1. 题目

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.

2. 思路

遍历组合即可，递归处理。

3. 代码

耗时：3ms

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> ret;
        for (int i = 0; i < digits.size(); i++) {
            if (digits[i] < '2' || digits[i] > '9') return ret;
        }
        if (digits.length() == 0) {
            return ret;
        } else {
            int k = digits[0] - '0';
            for (int i = 0; i < lens[k]; i++) {
                char ch = m[k][i];
                vector<string> subs = letterCombinations(digits.substr(1));
                if (subs.size() == 0) {
                    string s;
                    s += ch;
                    ret.push_back(s);
                } else {
                    for (int j = 0; j < subs.size(); j++) {
                        string s;
                        s += ch;
                        s += subs[j];
                        ret.push_back(s);
                    }
                }
            }
        }
        return ret;
    }
    
private:
    static const int lens[10];
    static const char m[10][4];
};

const int Solution::lens[10] = {0, 0, 3, 3, 3, 3, 3, 4, 3, 4};
const char Solution::m[10][4] = {{0, 0, 0, 0}, {0, 0, 0, 0}, {'a', 'b', 'c', 0}, {'d', 'e', 'f', 0},
                                {'g', 'h', 'i', 0}, {'j', 'k', 'l', 0}, {'m', 'n', 'o', 0},
                                {'p', 'q', 'r', 's'}, {'t', 'u', 'v', 'w'}, {'w', 'x', 'y', 'z'}};