Leetcode dfs Letter Combinations of a Phone Number

Letter Combinations of a Phone Number

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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;}