leetcode——17——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.

class Solution {public:    vector<string> letterCombinations(string digits) {        vector<string> ret;        if(digits == "")            return ret;                    ret.push_back("");                vector<string> dict(10); //0~9        dict[2] = "abc";        dict[3] = "def";        dict[4] = "ghi";        dict[5] = "jkl";        dict[6] = "mno";        dict[7] = "pqrs";        dict[8] = "tuv";        dict[9] = "wxyz";                for(int i = 0; i < digits.size(); i ++)        {            int size = ret.size();            for(int j = 0; j < size; j ++)            {                string cur = ret[0];                ret.erase(ret.begin());                for(int k = 0; k < dict[digits[i]-'0'].size(); k ++)                {                    ret.push_back(cur + dict[digits[i]-'0'][k]);                }            }        }        return ret;            }};