41. 缺失的第一个正数

<?php
class Solution {
    /**
     * 缺失的第一个正数
     * 输入: [3,4,-1,1]
     * 输出: 2
     *
     * 输入: [7,8,9,11,12]
     * 输出: 1
     * @param Integer[] $nums
     * @return Integer
     */
    function firstMissingPositive($nums) {
        if (!$nums) return 1;
        // $nums[$k - 1] = $k;
        foreach ($nums as $k => $v) {
            while ($v > 0 && $nums[$nums[$k] - 1] != $nums[$k]) {
                $tmp = $nums[$nums[$k] - 1];
                $nums[$nums[$k] - 1] = $nums[$k];
                $nums[$k] = $tmp;
            }
        }
        foreach ($nums as $k => $v) {
            if ($v === null) {
                return $nums[$k] + 1;
            }
        }
        return end($nums) + 1;
    }
}
// $nums = [3,4,-1,1];
$nums = [7,8,9,11,12];
$cls = new Solution();
$r = $cls->firstMissingPositive($nums);
echo $r;