# [Implement strStr()][title]

## Description

Implement [strStr()](http://www.cplusplus.com/reference/cstring/strstr/).

Return the index of the first occurrence of needle in haystack, or **-1** if needle is not part of haystack.

**Example 1:**

```
Input: haystack = "hello", needle = "ll"
Output: 2
```

**Example 2:**

```
Input: haystack = "aaaaa", needle = "bba"
Output: -1
```

**Clarification:**

What should we return when `needle` is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when `needle` is an empty string. This is consistent to C's [strstr()](http://www.cplusplus.com/reference/cstring/strstr/) and Java's [indexOf()](https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#indexOf(java.lang.String)).

Tags:** Two Pointers, String


## 思路

题意是从主串中找到子串的索引，如果找不到则返回-1，当子串长度大于主串，直接返回-1，然后我们只需要遍历比较即可。

```java
class Solution {
    public int strStr(String haystack, String needle) {
        int l1 = haystack.length(), l2 = needle.length();
        if (l1 < l2) return -1;
        for (int i = 0; ; i++) {
            if (i + l2 > l1) return -1;
            for (int j = 0; ; j++) {
                if (j == l2) return i;
                if (haystack.charAt(i + j) != needle.charAt(j)) break;
            }
        }
    }
}
```


## 结语

如果你同我一样热爱数据结构、算法、LeetCode，可以关注我 GitHub 上的 LeetCode 题解：[awesome-java-leetcode][ajl]



[title]: https://leetcode.com/problems/implement-strstr
[ajl]: https://github.com/Blankj/awesome-java-leetcode