php 实现二分查找的两种方法

非递归实现

<?php
/**
 * Created by phpStorm.
 * User: BinWei
 * Date: 2019/6/1
 * Time: 19:18
 */


/**
 * @description
 * @param $list
 * @param $item
 * @return float|int|null
 * @author BinWei
 */
function binary_search($list, $item)
{
    $low = 0;
    $high = count($list) - 1;

    while ($low <= $high) { // 只要范围没有缩小到只包含一个元素,就检查中间元素
        if (($low + $high) % 2 == 0) { // 判断是否是偶数
            $mid = ($low + $high) / 2; // 取中间值
        } else {
            $mid = floor(($low + $high) / 2); // 向下取整
        }
        $guess = $list[$mid];
        if ($guess == $item) {
            return $mid;
        }

        if ($guess > $item) {
            $high = $mid - 1;
        }

        if ($guess < $item) {
            $low = $mid + 1;
        }
    }

    return null; // 没有找到元素
}

$list = [0, 1, 2, 3, 4, 5, 10];
var_dump(binary_search($list, 10));

以递归方式实现

<?php
/**
 * Created by phpStorm.
 * User: BinWei
 * Date: 2019/6/1
 * Time: 19:09
 */


/**
 * @description
 * @param $list
 * @param $low
 * @param $high
 * @param $item
 * @return float|int|null
 * @author BinWei
 */
function binary_search($list, $low, $high, $item)
{
    if ($low <= $high) {
        if (($low + $high) % 2 == 0) {
            $mid = ($low + $high) / 2;
        } else {
            $mid = floor(($low + $high) / 2);
        }

        if ($list[$mid] == $item) {
            return $mid;
        }

        if ($list[$mid] > $item) {
            return binary_search($list, $low, $mid - 1, $item);
        }

        if ($list[$mid] < $item) {
            return binary_search($list, $mid + 1, $high, $item);
        }

    } else {
        return null;
    }
}


$list = [100, 102, 202, 302, 402, 502, 1011];
var_dump(binary_search($list, 0, count($list) - 1, 502));