14. 电话号码的字母组合

<?php
class Solution {
    public $res = [];
    public $str = "";
    public $array = [
        '2'=> ['a', 'b', 'c'],
        '3'=> ['d', 'e', 'f'],
        '4'=> ['g', 'h', 'i'],
        '5'=> ['j', 'k', 'l'],
        '6'=> ['m', 'n', 'o'],
        '7'=> ['p', 'q', 'r', 's'],
        '8'=> ['t', 'u', 'v'],
        '9'=> ['w', 'x', 'y', 'z'],
    ];
    public function letterCombinations($digits) {
        if (!$digits) return [];
        $this->_dfs($digits, 0);
        return $this->res;
    }
    private function _dfs($digits, $step = 0) {
        if ($step == strlen($digits)) {
            $this->res[] = $this->str;
            return;
        }
        $key = substr($digits, $step, 1);
        $chars = $this->array[$key];
        foreach ($chars as $v) {
            $this->str .= $v;
            $this->_dfs($digits, $step + 1);
            $this->str = substr($this->str, 0, strlen($this->str) - 1);
        }
    }
}
$cls = new Solution();
$cls->letterCombinations('23');
print_r($cls->res);