{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 前言\n",
    "在Scala中，List是一个高效的数据结构，尤其对于很多递归算法尤其重要。它可以容易在头部添加元素，也可以得到列表的尾部。"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 列表的最后一个元素"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mlist1\u001b[0m: \u001b[32mList\u001b[0m[\u001b[32mInt\u001b[0m] = \u001b[33mList\u001b[0m(\u001b[32m1\u001b[0m, \u001b[32m1\u001b[0m, \u001b[32m2\u001b[0m, \u001b[32m3\u001b[0m, \u001b[32m5\u001b[0m, \u001b[32m8\u001b[0m)"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "val list1 = List(1, 1, 2, 3, 5, 8)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mfunction \u001b[36mlastBuildin\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "def lastBuildin[A](ls: List[A]): A = ls.last"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mfunction \u001b[36mlastRecursive\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "def lastRecursive[A](ls: List[A]): A = ls match {\n",
    "    case h :: Nil => h\n",
    "    case _ :: tail => lastRecursive(tail)\n",
    "    case _ => throw new NoSuchElementException\n",
    "}"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mres3\u001b[0m: \u001b[32mInt\u001b[0m = \u001b[32m8\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "lastBuildin(list1)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mres4\u001b[0m: \u001b[32mInt\u001b[0m = \u001b[32m8\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "lastRecursive(list1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 列表的倒数第二个元素"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mfunction \u001b[36mpenultimateBuildin\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "def penultimateBuildin[A](ls: List[A]): A =\n",
    "    if(ls.isEmpty) throw new NoSuchElementException\n",
    "    else ls.init.last"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mfunction \u001b[36mpenultimateRecursive\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "def penultimateRecursive[A](ls: List[A]): A = ls match {\n",
    "    case h :: _ :: Nil => h\n",
    "    case _ :: tail => penultimateRecursive(tail)\n",
    "    case _ => throw new NoSuchElementException\n",
    "}"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mres7\u001b[0m: \u001b[32mInt\u001b[0m = \u001b[32m5\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "penultimateBuildin(list1)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mres8\u001b[0m: \u001b[32mInt\u001b[0m = \u001b[32m5\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "penultimateRecursive(list1)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  }
 ],
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