Leetcode–Letter Combinations of a Phone Number

The Problem:

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"].


  1. Similar to Combinations, just list every combination.
  2. I tried using StringBuffer instead of String, the speed doesn’t show a big difference, but code gets less concise. Anyway, I think StringBuffer is a better choice and will show advantage when test cases gets bigger.

Java Solution

public class Solution {
    public List<String> letterCombinations(String digits) {
        String[] letters= {" ","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        List<String> res = new ArrayList<String>();
        if(digits==null || digits.length()==0){
            return res;
        for(int i = 0; i < digits.length(); i++){
                List<String> temp = new ArrayList<String>();
                for(int j = 0; j < letters[digits.charAt(i)-'0'].length(); j++){
                        for(String s: res){
                res = temp;
        return res;

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