Letter Combinations of a Phone Number

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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"].

递归，深度优先搜索

public class Solution {public List<String> letterCombinations(String digits) {List<String> res = new ArrayList<String>();String strs[] = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs","tuv", "wxyz" };if (digits == null || digits.length() == 0) {res.add("");return res;}if (digits.length() == 1) {int k = Integer.parseInt(digits);if (k < 2) {res.add("");return res;}String s = strs[k];for (int i = 0; i < s.length(); i++) {res.add(s.charAt(i) + "");}return res;}List<String> temp = letterCombinations(digits.substring(1));String str = strs[Integer.parseInt(digits.substring(0, 1))];if (str == "") {return temp;}for (int i = 0; i < str.length(); i++) {for (int j = 0; j < temp.size(); j++) {res.add(str.charAt(i) + temp.get(j));}}return res;}}

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