[LeetCode]Letter Combinations of a Phone Number

题目描述:(链接)

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：

递归方式，深度优先搜索

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> result;
        if (digits == "") return result;
        vector<string> keyboard {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        string midResult(digits.size(), '\0');
        dfs(digits, 0, midResult, keyboard, result);
        return result;
    }
private:
    void dfs(const string &digits, int index, string &midResult, const vector<string> &keyboard, vector<string> &result) {
        if (index == digits.size()) {
            result.push_back(midResult);
            return;
        }
        
        char c = digits[index];
        for (int i = 0; i < keyboard[c - '0'].size(); ++i) {
            midResult[index] = keyboard[c - '0'][i];
            dfs(digits, index + 1, midResult, keyboard, result);
        }
    }
};