Letter Combinations of a Phone Number

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.

用dfs解决

class Solution { public: vector<string> letterCombinations(string digits) { vector<string>ans; dfs(digits,,ans); return ans; } void dfs(string digits,int start,string s,vector<string>&ans){ if(start == digits.size()){ if(s != ) ans.push_back(s); return; } int cnt = digits[start]-'2'; for(int i = 0; i < 4; i++) if(letter[cnt][i] != '!') dfs(digits,start+1,s+letter[cnt][i],ans); } char letter[8][4] = { {'a','b','c','!'},{'d','e','f',{'g','h','i',{'j','k','l',{'m','n','o',{'p','q','r','s'},{'t','u','v',{'w','x','y','z'} }; };


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