LeetCode --Letter Combinations of a Phone Number

题目：

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.

解读：给与一组由0-9组成的整数串，每个整数代表26个字母中的若干个，参考手机上的标准。给出由这些整数代表的字母组成的字母串的合集。

思路：采用循环迭代的方式，第一个数字的字母与第二个数字的字母进行组合，得出的结果和第三个数字的字母进行组合，得出的结果和第四个数字的字母进行组合....

注意点：字符串为0的时候返回空vector。否则，进行vector初始化为{“”}。

代码

class Solution {public:    vector<string> letterCombinations(string digits) {        vector<string> answer;        string letter[10] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};        if(digits.length() == 0) return answer;       answer = {""};        for(int i = 0; i<digits.length(); i++){            vector<string> t;            string l = letter[digits[i]-48];            for(int j = 0; j < answer.size(); j++){                if(l == "") break;                else {                    for(int k = 0; k < l.length(); k++){                        t.push_back(answer[j] + l[k]);                    }                }            }            answer = t;        }        return answer;    }};