leetcode Letter Combinations of a Phone Number

题目连接

https://leetcode.com/problems/letter-combinations-of-a-phone-number/  

Letter Combinations of a Phone Number

Description

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.

dfs。。

class Solution {
public:
	vector<string> letterCombinations(string digits) {
		init();
		ans.clear();
		n = digits.length();
		if (!n) return ans;
		dfs(0, "", digits);
		return ans;
	}
private:
	size_t n;
	map<int, string> A;
	vector<string>ans;
	void init() {
		A[2] = "abc";
		A[3] = "def";
		A[4] = "ghi";
		A[5] = "jkl";
		A[6] = "mno";
		A[7] = "pqrs";
		A[8] = "tuv";
		A[9] = "wxyz";
	}
	void dfs(int cur, string x, string digits) {
		if (cur == n) { ans.push_back(x); return; }
		int d = digits[cur] - '0';
		for (size_t i = 0; i < A[d].size(); i++) {
			dfs(cur + 1, x + A[d][i], digits);
		}
	}
};