Leetcode 18 letter combinations of a phone number

Given a digit string, return all possible letter combinations that the number coshould 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 cocould be in any order you want.

Idea: use recursion.

public class Solution {List<String> result = new ArrayList<String>();String[] string = new String[] { " ", "", "abc", "def", "ghi", "jkl", "mno","pqrs", "tuv", "wxyz" };public List<String> letterCombinations(String digits) {result.add(new String(""));if (digits.length() == 0)return result;letterOfCombinations(digits);return result;}public void letterOfCombinations(String digits) {int tmp = (int) (digits.charAt(0) - '0');int len = result.size();int i, j;for (i = 0; i < string[tmp].length() - 1; i++) {for (j = 0; j < len; j++) {result.add(new String(result.get(j)));}}for (i = 0; i < result.size(); i++) {j = i / len;String str = result.get(i) + string[tmp].charAt(j);result.set(i, str);}if (digits.length() == 1) return;letterOfCombinations(digits.substring(1));}}

Leetcode 18 letter combinations of a phone number