問題描述：

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.

示例：

Input:Digit string "23"Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
問題分析：
這可以看作一個組合問題
1.設置一個結果集result值為空
2.對每個digit進行遍歷，將其對應的字符添加分別與result結果集中的每個元素組合即可
下面是具體實現：
class Solution {public:    vector<string> letterCombinations(string digits) {        vector<string> result;        	if(digits.empty()) return vector<string>();        	vector<string> v = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};        	result.push_back("");        	        	for(int i = 0; i  < digits.size(); i++)        	{        		int num = digits[i] - '0';        		if(num < 0 || num > 9) break;        		string candidate = v[num];        		if(candidate.empty()) continue;//出現數字0和1         		vector<string> tmp;				// 將result和candidate的字符串組合一下 				for(int j = 0 ; j < candidate.size() ; j++) {            		for(int k = 0 ; k < result.size() ; k++) {                		tmp.push_back(result[k] + candidate[j]);            		}        		}				result.swap(tmp); 			}			return result;    }};好了，就先記這么多吧~