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.

s思路：
1. 需要列举所有情况，一般都是for+recursive来实现backtracking。这道题，相当于排列组合。

class Solution {
private:
    vector<string> letters={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
public:

    void helper(vector<string>&res,string&digits,string cur,int idx){
        if(idx==digits.size()){
            res.push_back(cur);
            return;
        }
        for(int i=0;i<letters[digits[idx]-'0'].size();i++){
            cur+=letters[digits[idx]-'0'][i];
            helper(res,digits,cur,idx+1);
            cur.pop_back();//跟pop相关的操作不需要跟操作符！  
        }
    }
    vector<string> letterCombinations(string digits) {
        //

        if(digits.empty()) return vector<string>(0);
        vector<string> res;
        helper(res,digits,"",0);
        return res;
    }
};