# Technical interview questions

<table>
   <tr>
      <td>⚠️</td>
      <td>
         The answers here are given by the community. Be careful and double check the answers before using them. <br>
         If you see an error, please create a PR with a fix
      </td>
   </tr>
</table>

The list is based on [this post](https://medium.com/data-science-insider/technical-data-science-interview-questions-f61cd9cf218?source=friends_link&sk=01f4de0de746d28fe714d92a1e91e190)


## Table of contents

* [SQL](#sql)
* [Coding (Python)](#coding-python)
* [Algorithmic Questions](#algorithmic-questions)

<br/>

## SQL

Suppose we have the following schema with two tables: Ads and Events

* Ads(ad_id, campaign_id, status)
* status could be active or inactive
* Events(event_id, ad_id, source, event_type, date, hour)
* event_type could be impression, click, conversion

<img src="img/schema.png" />


Write SQL queries to extract the following information:

**1)** The number of active ads.

```sql
SELECT count(*) FROM Ads WHERE status = 'active';
```

<br/>


**2)** All active campaigns. A campaign is active if there’s at least one active ad.

```sql
SELECT DISTINCT a.campaign_id
FROM Ads AS a
WHERE a.status = 'active';
```

<br/>

**3)** The number of active campaigns.

```sql
SELECT COUNT(DISTINCT a.campaign_id)
FROM Ads AS a
WHERE a.status = 'active';
```

<br/>

**4)** The number of events per each ad — broken down by event type.

<img src="img/sql_4_example.png" />

```sql
SELECT e.ad_id, e.event_type, count(*) as "count"
FROM Events AS e
GROUP BY e.ad_id, e.event_type
ORDER BY e.ad_id, "count" DESC;
```

<br/>

**5)** The number of events over the last week per each active ad — broken down by event type and date (most recent first).

<img src="img/sql_5_example.png" />

```sql
SELECT a.ad_id, e.event_type, e.date, count(*) as "count"
FROM Ads AS a
  JOIN Events AS e
      ON a.ad_id = e.ad_id
WHERE a.status = 'active'
   AND e.date >= DATEADD(week, -1, GETDATE())
GROUP BY a.ad_id, e.event_type, e.date
ORDER BY e.date ASC, "count" DESC;
```

<br/>

**6)** The number of events per campaign — by event type.

<img src="img/sql_6_example.png" />


```sql
SELECT a.campaign_id, e.event_type, count(*) as count
FROM Ads AS a
  INNER JOIN Events AS e
    ON a.ad_id = e.ad_id
GROUP BY a.campaign_id, e.event_type
ORDER BY a.campaign_id, "count" DESC
```

<br/>

**7)** The number of events over the last week per each campaign and event type — broken down by date (most recent first).

<img src="img/sql_7_example.png" />

```sql
-- for Postgres

SELECT a.campaign_id AS camp_id, e.event_type, e.date, count(*)
FROM Ads AS a
  INNER JOIN Events AS e
    ON a.ad_id = e.ad_id
WHERE e.date >= DATEADD(week, -1, GETDATE())
GROUP BY a.campaign_id, e.event_type, e.date
ORDER BY a.campaign_id, e.date DESC, "count" DESC;
```

<br/>

**8)** CTR (click-through rate) for each ad. CTR = number of clicks / number of impressions.

<img src="img/sql_8_example.png" />

```sql
-- for Postgres

SELECT impressions_clicks_table.ad_id,
       (impressions_clicks_table.clicks * 100 / impressions_clicks_table.impressions)::FLOAT || '%' AS CTR
FROM
  (
  SELECT e.ad_id,
         SUM(CASE e.event_type WHEN 'impression' THEN 1 ELSE 0 END) impressions,
         SUM(CASE e.event_type WHEN 'click' THEN 1 ELSE 0 END) clicks
  FROM Events AS e
  GROUP BY e.ad_id
  ) AS impressions_clicks_table
ORDER BY impressions_clicks_table.ad_id;
```

<br/>

**9)** CVR (conversion rate) for each ad. CVR = number of conversions / number of clicks.

<img src="img/sql_9_example.png" />

```sql
-- for Postgres

SELECT conversions_clicks_table.ad_id,
       (conversions_clicks_table.conversions * 100 / conversions_clicks_table.clicks)::FLOAT || '%' AS CVR
FROM
  (
  SELECT e.ad_id,
         SUM(CASE e.event_type WHEN 'conversion' THEN 1 ELSE 0 END) conversions,
         SUM(CASE e.event_type WHEN 'click' THEN 1 ELSE 0 END) clicks
  FROM Events AS e
  GROUP BY e.ad_id
  ) AS conversions_clicks_table
ORDER BY conversions_clicks_table.ad_id;
```

<br/>

**10)** CTR and CVR for each ad broken down by day and hour (most recent first).

<img src="img/sql_10_example.png" />


```sql
-- for Postgres

SELECT conversions_clicks_table.ad_id,
       conversions_clicks_table.date,
       conversions_clicks_table.hour,
       (impressions_clicks_table.clicks * 100 / impressions_clicks_table.impressions)::FLOAT || '%' AS CTR,
       (conversions_clicks_table.conversions * 100 / conversions_clicks_table.clicks)::FLOAT || '%' AS CVR
FROM
  (
  SELECT e.ad_id, e.date, e.hour,
         SUM(CASE e.event_type WHEN 'conversion' THEN 1 ELSE 0 END) conversions,
         SUM(CASE e.event_type WHEN 'click' THEN 1 ELSE 0 END) clicks,
         SUM(CASE e.event_type WHEN 'impression' THEN 1 ELSE 0 END) impressions
  FROM Events AS e
  GROUP BY e.ad_id, e.date, e.hour
  ) AS conversions_clicks_table
ORDER BY conversions_clicks_table.ad_id, conversions_clicks_table.date DESC, conversions_clicks_table.hour DESC, "CTR" DESC, "CVR" DESC;
```

<br/>

**11)** CTR for each ad broken down by source and day

<img src="img/sql_11_example.png" />


```sql
-- for Postgres

SELECT conversions_clicks_table.ad_id,
       conversions_clicks_table.date,
       conversions_clicks_table.source,
       (impressions_clicks_table.clicks * 100 / impressions_clicks_table.impressions)::FLOAT || '%' AS CTR
FROM
  (
  SELECT e.ad_id, e.date, e.source,
         SUM(CASE e.event_type WHEN 'click' THEN 1 ELSE 0 END) clicks,
         SUM(CASE e.event_type WHEN 'impression' THEN 1 ELSE 0 END) impressions
  FROM Events AS e
  GROUP BY e.ad_id, e.date, e.source
  ) AS conversions_clicks_table
ORDER BY conversions_clicks_table.ad_id, conversions_clicks_table.date DESC, conversions_clicks_table.source, "CTR" DESC;
```

<br/>


## Coding (Python)

**1) FizzBuzz.** Print numbers from 1 to 100

* If it’s a multiplier of 3, print “Fizz”
* If it’s a multiplier of 5, print “Buzz”
* If both 3 and 5 — “Fizz Buzz"
* Otherwise, print the number itself

Example of output: 1, 2, Fizz, 4, Buzz, Fizz, 7, 8, Fizz, Buzz, 11, Fizz, 13, 14, Fizz Buzz, 16, 17, Fizz, 19, Buzz, Fizz, 22, 23, Fizz, Buzz, 26, Fizz, 28, 29, Fizz Buzz, 31, 32, Fizz, 34, Buzz, Fizz, ...

```python
for i in range(1, 101):
    if i % 3 == 0 and i % 5 == 0:
        print('Fizz Buzz')
    elif i % 3 == 0:
        print('Fizz')
    elif i % 5 == 0:
        print('Buzz')
    else:
        print(i)
```

<br/>

**2) Factorial**. Calculate a factorial of a number

* `factorial(5)` = 5! = 1 * 2 * 3 * 4 * 5 = 120
* `factorial(10)` = 10! = 1 * 2 * 3 * 4 * 5 * 6 * 7 * 8 * 9 * 10 = 3628800

```python
def factorial(n):
    result = 1
    for i in range(2, n + 1):
        result *= i
    return result
```

We can also write this function using recursion:

```python
def factorial(n: int):
    if n == 0 or n == 1:
        return 1
    else:
        return n * factorial(n - 1)
```


<br/>

**3) Mean**. Compute the mean of number in a list

* `mean([4, 36, 45, 50, 75]) = 42`
* `mean([]) = NaN` (use `float('NaN')`)

<img src="img/formula_mean.png" />

```python
def mean(numbers):
    if len(numbers) > 0:
        return sum(numbers) / len(numbers)
    return float('NaN')
```

<br/>

**4) STD**. Calculate the standard deviation of elements in a list.

* `std([1, 2, 3, 4]) = 1.29`
* `std([1]) = NaN` (use `float('NaN')`)
* `std([]) = NaN`

<img src="img/formula_std.png" />

```python
from math import sqrt
from statistics import mean

def std_dev(numbers):
    if len(numbers) > 1:
        avg = mean(numbers)
        var = sum((i - avg) ** 2 for i in numbers) / (len(numbers) - 1)
        std = sqrt(var)
        return std
    return float('NaN')
```

<br/>

**5) RMSE**. Calculate the RMSE (root mean squared error) of a model. The function takes in two lists: one with actual values, one with predictions.

* `rmse([1, 2], [1, 2]) = 0`
* `rmse([1, 2, 3], [3, 2, 1]) = 1.63`

<img src="img/formula_rmse.png" />

```python
import math

def rmse(y_true, y_pred):
    assert len(y_true) == len(y_pred), 'different sizes of the arguments'
    squares = sum((x - y)**2 for x, y in zip(y_true, y_pred))
    return math.sqrt(squares / len(y_true))
```

<br/>

**6) Remove duplicates**. Remove duplicates in list. The list is not sorted and the order of elements from the original list should be preserved.

* `[1, 2, 3, 1]` ⇒ `[1, 2, 3]`
* `[1, 3, 2, 1, 5, 3, 5, 1, 4]` ⇒ `[1, 3, 2, 5, 4]`

```python
def remove_duplicates(lst):
    new_list = []
    mentioned_values = set()
    for elem in lst:
        if elem not in mentioned_values:
            new_list.append(elem)
            mentioned_values.add(elem)
    return new_list

# The above solution checks the values into a set and it is O(1) efficient using
# a few of lines.
# A shorter solution follows: it is O(n^2) but can be fine when lst has no "too
# many elements" - the quantity depends by the running box.
def remove_duplicates2(lst):
    new_list = []
    for elem in lst:
        if elem not in new_list:
            new_list.append(elem)
    return new_list
```

<br/>

**7) Count**. Count how many times each element in a list occurs.

`[1, 3, 2, 1, 5, 3, 5, 1, 4]` ⇒  
* 1: 3 times
* 2: 1 time
* 3: 2 times
* 4: 1 time
* 5: 2 times

```python
numbers = [1, 3, 2, 1, 5, 3, 5, 1, 4]
counter = dict()
for elem in numbers:
    counter[elem] = counter.get(elem, 0) + 1
```
or
```python
from collections import Counter

numbers = [1, 3, 2, 1, 5, 3, 5, 1, 4]
counter = Counter(numbers)
```

<br/>

**8) Palindrome**. Is string a palindrome? A palindrome is a word which reads the same backward as forwards.

* “ololo” ⇒ Yes
* “cafe” ⇒ No

```python
def is_palindrome(s):
    return s == s[::-1]
```
or
```python
def is_palindrome(s):
    for i in range(len(s) // 2):
        if s[i] != s[-i - 1]:
            return False
    return True
```

<br/>

**9) Counter**. We have a list with identifiers of form “id-SITE”. Calculate how many ids we have per site.

<img src="img/counter_1.png" />

```python
def counter(lst):
    ans = {}
    for i in lst:
        site = i[-2:]
        ans[site] = ans.get(site, 0) + 1
    return ans
```

<br/>

**10) Top counter**. We have a list with identifiers of form “id-SITE”. Show the top 3 sites. You can break ties in any way you want.

<img src="img/counter_2_top.png" />

```python
def top_counter(lst):
    site_dict = counter(lst)  # using last problem's solution
    top_keys = sorted(site_dict, reverse=True, key=site_dict.get)[:3]
    return {key: site_dict[key] for key in top_keys}
```

<br/>

**11) RLE**. Implement RLE (run-length encoding): encode each character by the number of times it appears consecutively.

* `'aaaabbbcca'` ⇒ `[('a', 4), ('b', 3), ('c', 2), ('a', 1)]`
* (note that there are two groups of 'a')

```python
def rle(s):
    ans, cur, num = [], None, 0
    for i in range(len(s)):
        if i == 0:
            cur, num = s[i], 1
        elif cur != s[i]:
            ans.append((cur, num))
            cur, num = s[i], 1
        else:
            num += 1
        if i == len(s) - 1:
            ans.append((cur, num))
    return ans
```

Using itertools.groupby
```python
import itertools

def rle(s):
    return [(l, len(list(g))) for l, g in itertools.groupby(s)]
```

<br/>

**12) Jaccard**. Calculate the Jaccard similarity between two sets: the size of intersection divided by the size of union.

* `jaccard({'a', 'b', 'c'}, {'a', 'd'}) = 1 / 4`

<img src="img/formula_jaccard.png" />

```python
def jaccard(a, b):
    return len(a & b) / len(a | b)
```

<br/>

**13) IDF**. Given a collection of already tokenized texts, calculate the IDF (inverse document frequency) of each token.

* input example: `[['interview', 'questions'], ['interview', 'answers']]`

<img src="img/formula_idf_1.png" />

Where:

* t is the token,
* n(t) is the number of documents that t occurs in,
* N is the total number of documents

```python
from math import log10

def idf1(docs):
    docs = [set(doc) for doc in docs]
    n_tokens = {}
    for doc in docs:
        for token in doc:
            n_tokens[token] = n_tokens.get(token, 0) + 1
    ans = {}
    for token in n_tokens:
        ans[token] = log10(len(docs) / (1 + n_tokens[token]))
    return ans
```

```python
import math

def idf2(docs):
    n_docs = len(docs)

    docs = [set(doc) for doc in docs]
    all_tokens = set.union(*docs)

    idf_coefficients = {}
    for token in all_tokens:
        n_docs_w_token = sum(token in doc for doc in docs)
        idf_c = math.log10(n_docs / (1 + n_docs_w_token))
        idf_coefficients[token] = idf_c

    return idf_coefficients
```

<br/>

**14) PMI**. Given a collection of already tokenized texts, find the PMI (pointwise mutual information) of each pair of tokens. Return top 10 pairs according to PMI.

* input example: `[['interview', 'questions'], ['interview', 'answers']]`

PMI is used for finding collocations in text — things like “New York” or “Puerto Rico”. For two consecutive words, the PMI between them is:

<img src="img/formula_pmi_1.png" />

The higher the PMI, the more likely these two tokens form a collection. We can estimate PMI by counting:

<img src="img/formula_pmi_2.png" />

Where:
* N is the total number of tokens in the text,
* c(t1, t2) is the number of times t1 and t2 appear together,
* c(t1) and c(t2) — the number of times they appear separately.

```python
import math

def pmi(docs):
    n_pairs = {}
    n_tokens = {}

    docs = [tuple(doc) for doc in docs]
    for doc in docs:
        for token in doc:
            n_tokens[token] = n_tokens.get(token, 0) + 1

        n_pairs[doc] = n_pairs.get(doc, 0) + 1
        ans = {}
        for pair in n_pairs:
            ans[pair] = math.log2(n_pairs[pair] * (sum(n_tokens.values())) / (n_tokens[pair[0]] * n_tokens[pair[1]]))

        srt = sorted(ans.items(), key=lambda x: x[1], reverse=True)[:10]

    return srt
```


<br/>

## Algorithmic Questions

**1) Two sum**. Given an array and a number N, return True if there are numbers A, B in the array such that A + B = N. Otherwise, return False.

* `[1, 2, 3, 4], 5` ⇒ `True`
* `[3, 4, 6], 6` ⇒ `False`

Brute force, O(n<sup>2</sup>):

```python
def two_sum(numbers, target):
    n = len(numbers)

    for i in range(n):
        for j in range(i + 1, n):
            if numbers[i] + numbers[j] == target:
                return True

    return False
```

Linear, O(n):

```python
def two_sum(numbers, target):
    index = {num: i for (i, num) in enumerate(numbers)}

    n = len(numbers)

    for i in range(n):
        a = numbers[i]
        b = target - a

        if b in index:
            j = index[b]
            if i != j:
                return True

    return False
```

Using itertools.combinations
```python
from itertools import combinations

def two_sum(numbers, target):
    for elem in combinations(numbers, 2):
        if elem[0] + elem[1] == target:
            return True
    return False
```


<br/>

**2) Fibonacci**. Return the n-th Fibonacci number, which is computed using this formula:

* F(0) = 0
* F(1) = 1
* F(n) = F(n-1) + F(n-2)
* The sequence is: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

```python
def fibonacci1(n):
    '''naive, complexity = O(2 ** n)'''
    if n == 0 or n == 1:
        return n
    else:
        return fibonacci1(n - 1) + fibonacci1(n - 2)
```

```python
def fibonacci2(n):
    '''dynamic programming, complexity = O(n)'''
    base1, base2 = 0, 1
    for i in range(n):
        base1, base2 = base2, base1 + base2
    return base1
```

```python
def fibonacci3(n):
    '''matrix multiplication, complexity = O(log(n))'''
    def mx_mul(m1, m2):
        ans = [[0 for i in range(len(m2[0]))] for j in range(len(m1))]
        for i in range(len(m1)):
            for j in range(len(m2[0])):
                for k in range(len(m2)):
                    ans[i][j] += m1[i][k] * m2[k][j]
        return ans
    def pow(a, b):
        ans = [[1, 0], [0, 1]]
        while b > 0:
            if b % 2 == 1:
                ans = mx_mul(ans, a)
            a = mx_mul(a, a)
            b //= 2
        return ans
    ans = mx_mul(pow([[1, 1], [1, 0]], n), [[1], [0]])[1][0]
    return ans
```

Memoization with a dictionary

```python
memo = {0: 0, 1: 1}

def fibonacci4(n):
    '''Top down + memorization (dictionary), complexity = O(n)'''
    if n not in memo:
        memo[n] = fibonacci4(n-1) + fibonacci4(n-2)
    return memo[n]
```

Memoization with `lru_cache`

```python
from functools import lru_cache

@lru_cache()
def fibonacci4(n):
    if n == 0 or n == 1:
        return n
    return fibonacci4(n - 1) + fibonacci4(n - 2)
```

```python
def fibonacci5(n):
    '''Top down + memorization (list), complexity = O(n) '''
     if n == 1:
        return 1
    dic = [-1 for i in range(n)]
    dic[0], dic[1] = 1, 2
    def helper(n, dic):
        if dic[n] < 0:
            dic[n] = helper(n-1, dic) + helper(n-2, dic)
        return dic[n]
    return helper(n-1, dic)
```



<br/>

**3) Most frequent outcome**. We have two dice of different sizes (D1 and D2). We roll them and sum their face values. What are the most probable outcomes?

* 6, 6 ⇒ [7]
* 2, 4 ⇒ [3, 4, 5]

```python
def most_frequent_outcome(d1, d2):
    len_ans = abs(d1 - d2) + 1
    mi = min(d1, d2)
    ans = [mi + i for i in range(1, len_ans + 1)]
    return ans
```

<br/>

**4) Reverse a linked list**. Write a function for reversing a linked list.

* The definition of a list node: `Node(value, next)`
* Example: `a -> b -> c` ⇒ `c -> b -> a`

```python
def reverse_ll(head):
    if head.next is not None:
        last = None
        point = head
        while point is not None:
            point.next, point, last = last, point.next, point
```

<br/>

**5) Flip a binary tree**. Write a function for rotating a binary tree.

* The definition of a tree node: `Node(value, left, right)`

<img src="img/flip_binary_tree.png" />

```python
def flip_bt(head):
    if head is not None:
        head.left, head.right = head.right, head.left
        flip_bt(head.left)
        flip_bt(head.right)
```

<br/>

**6) Binary search**. Return the index of a given number in a sorted array or -1 if it’s not there.

* `[1, 4, 6, 10], 4` ⇒ `1`
* `[1, 4, 6, 10], 3` ⇒ `-1`

```python
def binary_search(lst, num):
    left, right = -1, len(lst)
    while right - left > 1:
        mid = (left + right) // 2
        if lst[mid] >= num:
            right = mid
        else:
            left = mid
    if right < 0 or right >= len(lst) or lst[right] != num:
        return -1
    else:
        return right
```

<br/>

**7) Deduplication**. Remove duplicates from a sorted array.

* `[1, 1, 1, 2, 3, 4, 4, 4, 5, 6, 6]` ⇒ `[1, 2, 3, 4, 5, 6]`

```python
def deduplication1(lst):
    '''manual'''
    ans = []
    last = None
    for i in lst:
        if last != i:
            ans.append(i)
            last = i
    return ans

def deduplication2(lst):
    # order is not guaranteed unless call sorted(list(set(lst))) to sort again
    return list(set(lst))
```

<br/>

**8) Intersection**. Return the intersection of two sorted arrays.

* `[1, 2, 4, 6, 10], [2, 4, 5, 7, 10]` ⇒ `[2, 4, 10]`

```python
def intersection1(lst1, lst2):
    '''reserves duplicates'''
    ans = []
    p1, p2 = 0, 0
    while p1 < len(lst1) and p2 < len(lst2):
        if lst1[p1] == lst2[p2]:
            ans.append(lst1[p1])
            p1, p2 = p1 + 1, p2 + 1
        elif lst1[p1] < lst2[p2]:
            p1 += 1
        else:
            p2 += 1
    return ans

def intersection2(lst1, lst2):
    '''removes duplicates'''
    # order is not guaranteed unless call sorted(...) to sort again
    return list(set(lst1) & set(lst2))
```

<br/>

**9) Union**. Return the union of two sorted arrays.

* `[1, 2, 4, 6, 10], [2, 4, 5, 7, 10]` ⇒ `[1, 2, 4, 5, 6, 7, 10]`

```python
def union1(lst1, lst2):
    '''reserves duplicates'''
    ans = []
    p1, p2 = 0, 0
    while p1 < len(lst1) or p2 < len(lst2):
        if lst1[p1] == lst2[p2]:
            ans.append(lst1[p1])
            p1, p2 = p1 + 1, p2 + 1
        elif lst1[p1] < lst2[p2]:
            ans.append(lst1[p1])
            p1 += 1
        else:
            ans.append(lst2[p2])
            p2 += 1
    return ans

def union2(lst1, lst2):
    '''removes duplicates'''
    # order is not guaranteed unless call sorted(...) to sort again
    return list(set(lst1) | set(lst2))
```

<br/>

**10) Addition**. Implement the addition algorithm from school. Suppose we represent numbers by a list of integers from 0 to 9:

* 12 is `[1, 2]`
* 1000 is `[1, 0, 0, 0]`

Implement the “+” operation for this representation

* `[1, 1] + [1]` ⇒ `[1, 2]`
* `[9, 9] + [2]` ⇒ `[1, 0, 1]`

```python
def addition(lst1, lst2):
    def list_to_int(lst):
        ans, base = 0, 1
        for i in lst[::-1]:
            ans += i * base
            base *= 10
        return ans
    val = list_to_int(lst1) + list_to_int(lst2)
    ans = [int(i) for i in str(val)]
    return ans

# another solution without int() and str() should be helpful
def addition2(lst1, lst2):
    if len(lst2) == 0 or lst2 == [0]:
        return lst1[:]
    elif len(lst1) == 0:
        return lst2[:]
    # lst1, lst2 not empty
    digit1, lst1rest = lst1[-1], lst1[:-1]
    digit2, lst2rest = lst2[-1], lst2[:-1]
    digit, remainder = divmod(digit1 + digit2, 10)
    lst = addition2(
        addition2(lst1rest, [digit]),  # recursively add digit to lst1
        lst2rest)  # and then continue to add lst2
    ans = lst + [remainder]  # add the remainder as last digit
    return ans
```

<br/>

**11) Sort by custom alphabet**. You’re given a list of words and an alphabet (e.g. a permutation of Latin alphabet). You need to use this alphabet to order words in the list.

Example:

* Words: `['home', 'oval', 'cat', 'egg', 'network', 'green']`
* Dictionary: `'bcdfghijklmnpqrstvwxzaeiouy'`

Output:

* `['cat', 'green', 'home', 'network', 'egg', 'oval']`

```python
def sort_by_custom_alphabet(dictionary, words):
    words = sorted(words, key = lambda word: [dictionary.index(c) for c in word])
    return words
```

<br/>

**12) Check if a tree is a binary search tree**. In BST, the element in the root is:

* Greater than or equal to the numbers on the left
* Less than or equal to the number on the right
* The definition of a tree node: `Node(value, left, right)`

```python
def check_is_bst(head, min_val=None, max_val=None):
    """Check whether binary tree is binary search tree

    Aside of the obvious node.left.val <= node.val <= node.right.val have to be
    fulfilled, we also have to make sure that there is NO SINGLE leaves in the
    left part of node have more value than the current node.
    """
    check_val = True
    check_left = True
    check_right = True

    if min_val:
        check_val = check_val and (head.val >= min_val)
        min_new = min(min_val, head.val)
    else:
        min_new = head.val

    if max_val:
        check_val = check_val and (head.val <= max_val)
        max_new = max(max_val, head.val)
    else:
        max_new = head.val

    if head.left:
        check_left = check_is_bst(head.left, min_val, max_new)

    if head.right:
        check_right = check_is_bst(head.right, min_new, max_val)

    return check_val and check_left and check_right
```

<br/>


**13) Maximum Sum Contiguous Subarray**. You are given an array `A` of length `N`, you have to find the largest possible sum of an Subarray, of array `A`.  
* `[-2, 1, -3, 4, -1, 2, 1, -5, 4]` gives `6` as largest sum (from the subarray `[4, -1, 2, -1]`

```python
from sys import maxsize
def max_sum_subarr(list1, size):
      """Use Kadane's Algorithm for a optimal solution
      Time Complexity: O(n)
      Desciption: Use one variable for current sum, and one for Overall sum at an index.
                  So here, the global_max will keep on updating the max sum at any index-1,
                  and curr_max will check the max value at an index.
                  And finally after iterating through the list, return the value of global_max variable which contains Maximum sum.
      """
    curr_max=list1[0]
    global_max=list1[0]
    for each in range(1, size):
        curr_max = max(list1[each], curr_max+list1[each])
        global_max = max(global_max, curr_max)
    return global_max

n = int(input())         
list1 = []
for i in range(0,n):
    num = int(input())
    list1.append(num)

print(max_sum_subarr(list1, len(list1)))
```

<br/>

**14) Three sum**. Given an array, and a target value, find all possible combinations of three distinct numbers such that the sum of these three distinct numbers is equal to the target value.

Example:

    Input: [12, 3, 1, 2, -6, 5, -8, 6], 0
    Output: [[-8, 2, 6], [-8, 3, 5], [-6, 1, 5]]

```python
def threeSum(array, target):
    array.sort()
    triplets = []
    
    for i in range(len(array) - 2):
        left = i + 1
        right = len(array) - 1
        while left < right:
            currentSum = array[i] + array[left] + array[right]
            if currentSum == target:
                triplets.append([array[i], array[left], array[right]])
                left += 1
                right -= 1
            elif currentSum > target:
                right -= 1
            elif currentSum < target:
                left += 1
    return triplets
```

**15) Find Duplicate in array** Given and array, find all duplicated value in the array

Example:
  
   input: [1,2,3,4,3,4,5]
   output: [3,4]
   
```python
a = [1,2,3,4,3,4,5]

def duplicates(a):
   seen = set()
   duplicated = set()
   for i in a:
      if i in seen:
         duplicated.add(i)
      else:
         seen.add(i)
   return duplicated
```