leetcode 17 Letter Combinations of a Phone Number

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

Input: "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.

 

思路：回溯法

class Solution {
private:
    // 创建映射map
    unordered_map<char, string> num2char {{'2', "abc"}, {'3', "def"}, {'4', "ghi"}, {'5', "jkl"},
                                          {'6', "mno"}, {'7', "pqrs"}, {'8', "tuv"}, {'9', "wxyz"}};
    
    void letterCombinations(vector<string> &res, string &combination, string digits, int index, int len) {
        if (index == len) {
            res.push_back(combination);
            return;
        }
        //遍历第index位可以选择的字符
        for (char c : num2char[digits[index]]) {
            combination.push_back(c);
            letterCombinations(res, combination, digits, index + 1, len);
            combination.pop_back();
        } 
        /*
        string current = num2char[digits[index]];
        for (int i = 0; i < current.length(); i++) {
            combination.push_back(current[i]);
            letterCombinations(res, combination, digits, index + 1, len);
            combination.pop_back();
        } */
    }
public:
    vector<string> letterCombinations(string digits) {
    
        int len = digits.length();
        vector<string> res;
        if (len == 0) {
            return res;
        }
        
        string combination;
        letterCombinations(res, combination, digits, 0, len);
        return res;
    }
};