【Leetcode】【Medium】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.

解题思路：

需要穷举不同中排列可能，典型需要回溯的思想；

可以选择递归或非递归的形式实现回溯；

递归：

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> lst;
        string ans;
        if (digits == "")
            return lst;
        Backtracking(lst, ans, digits, 0);
        return lst;
    }
    
    void Backtracking(vector<string> &lst, string ans, string digits, int idx) {
        if (idx == digits.size()) {
            lst.push_back(ans);
            return;
        }
        string cur_chars = d2l[digits[idx] - '0'];
        for (int i = 0; i < cur_chars.size(); ++i) 
            Backtracking(lst, ans + cur_chars[i], digits, idx + 1);
    }

private:
    vector<string> d2l = {
        " ", "", "abc", "def", "ghi", "jkl",
        "mno", "pqrs", "tuv", "wxyz"
    };
};

非递归：