LeetCode 17. Letter Combinations of a Phone Number（电话号码→字母组合）

题目描述：

    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.

例子：

Input: Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

注意：

    Although the above answer is in lexicographical order, your answer could be in any order you want.

分析：
    题意：电话机上，数字1-9各自对应一组不同的字母。给定按下的数字串，返回所有可能的字母组合。
    思路：题意需要返回所有结果（顺序任意），因此采用DFS搜索，直接上代码。

代码：

#include 

using namespace std;

class Solution {
private: 
	string str[10] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
	vector ans;

	void DFS(string &digits, string res, int n, int k){
		// Exceptional Case: 
		if(k == n){
			ans.push_back(res);
			return;
		}
		int len = str[digits[k] - '0'].length();
		for(int i = 0; i <= len - 1; i++){
			DFS(digits, res + str[digits[k] - '0'].substr(i, 1), n, k + 1);
		}
	}

public:
    vector letterCombinations(string digits) {
		int n = digits.length();
		// Exceptional Case: 
		if(n == 0){
			return vector();
		}
		// check if the digits contain 0 or 1
		for(int i = 0; i <= n - 1; i++){
			if(!(digits[i] >= '2' && digits[i] <= '9')){
				return vector();
			}
		}
		// DFS get answers
		ans.clear();
		DFS(digits, "", n, 0);
		return ans;
    }
};