LeetCode 17. Letter Combinations of a Phone Number（键盘字母）

原题网址：https://leetcode.com/problems/letter-combinations-of-a-phone-number/

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

方法：深度优先搜索。

public class Solution {
    char[][] dm = {
        {},
        {},
        {'a','b','c'},
        {'d','e','f'},
        {'g','h','i'},
        {'j','k','l'},
        {'m','n','o'},
        {'p','q','r','s'},
        {'t','u','v'},
        {'w','x','y','z'}
    };
    private void find(char[] digits, int step, char[] letters, List combinations) {
        if (step == digits.length) {
            combinations.add(new String(letters));
            return;
        }
        for(int i=0; i letterCombinations(String digits) {
        List combinations =new ArrayList<>();
        if (digits.length() == 0) return combinations;
        find(digits.toCharArray(), 0, new char[digits.length()], combinations);
        return combinations;
    }
}