159. 至多包含两个不同字符的最长子串

题意：

解题思路：

思路：滑动窗口思想
* [eceba]
* 当数组大于2个，比如[e => 2, c => 1, b => 1]
* 滑动数组：[e => 1, c => 1, b => 1] => [e => 1, c => 0, b => 1] => [e => 1, b => 1]

PHP代码实现：

class Solution {
    /**
     * @param String $s
     * @return Integer
     */
    function lengthOfLongestSubstringTwoDistinct($s) {
        $n = strlen($s);
        if($n <= 2) return $n;
        $map = [];
        $j = 0; $len = 2;
        for ($i = 0; $i < $n; $i++){
            //分组
            ++$map[$s[$i]];
            //当数组大于2个，比如[e => 2, c => 1, b => 1]
            //滑动数组：[e => 1, c => 1, b => 1] => [e => 1, c => 0, b => 1] => [e => 1, b => 1]
            while (count($map) > 2) {
                $map[$s[$j]]--;
                if($map[$s[$j]] == 0) unset($map[$s[$j]]);
                $j++;
            }
            //更新长度
            if (count($map) <= 2) $len = max($len, array_sum($map));
        }
        return $len;
    }
}