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<String> combinations) {
if (step == digits.length) {
combinations.add(new String(letters));
return;
}
for(int i=0; i<dm[digits[step]-'0'].length; i++) {
letters[step] = dm[digits[step]-'0'][i];
find(digits, step+1, letters, combinations);
}

}
public List<String> letterCombinations(String digits) {
List<String> combinations =new ArrayList<>();
if (digits.length() == 0) return combinations;
find(digits.toCharArray(), 0, new char[digits.length()], combinations);
return combinations;
}
}