[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"].

采用递归的方式。

class Solution {
public:
    //建立词典           0  1   2     3     4     5     6     7      8     9
    vector<string> dict{"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    vector<string> letterCombinations(string digits) {       
        vector<string> res;
        if(digits.size()==0) return res;
        help(digits, 0, string(""), res);
        return res;
        
    }
    //pos:当前数字的索引，pre：之前已经建立的字符串，res：保存的结果
    void help(string digits,int pos,string pre, vector<string>& res)
    {
        if(pos==digits.size())//digits遍历完成，保存结果
        {
            res.push_back(pre);
            return;
        }
        int num = digits[pos]-'0';
        for(int i=0;i<dict[num].size();i++)
        {
            string cur = pre+dict[num][i];
            help(digits, pos+1, cur, res);
            
        }
        
    }
};