一、题意

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。

给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

示例:

输入："23"
输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
说明:
尽管上面的答案是按字典序排列的，但是你可以任意选择答案输出的顺序。

二、解题

回溯法：

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'],
    ];
    /**
     * @param String $digits
     * @return String[]
     */
    function letterCombinations($digits) {
        if (!$digits) return [];
        $this->_dfs($digits, 0);
        return $this->res;
    }
    private function _dfs($digits, $step) {
        if ($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);
        }
    }
}