Leetcode dfs Letter Combinations of a Phone Number

Letter Combinations of a Phone Number

Total Accepted: 15964 Total
Submissions: 60700My Submissions

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.

题意：给定一串手机按键的数字串，求可能的字母串组合

思路：dfs

用一个字符串数组保存每个数字对应可选择的字母

dfs枚举每个数字的每种选择的字母

复杂度：时间O(3^n)，空间O(n)

string choices[] = {" ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
vector<string> res;
string _digits;
void dfs(string str, int cur){
	if(cur == _digits.size()){
		res.push_back(str);
		return ;
	}
	string choice = choices[_digits[cur] - '0'];
	for_each(choice.begin(), choice.end(), [&](char c){
		dfs(str + c, cur + 1);
		//回溯 --> 因为 str和 cur的值没改，所以不用
	});
}
vector<string> letterCombinations(string digits){
	_digits = digits;
	dfs("", 0);
	return res;
}