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,0,"",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'} 

    };
  };

Letter Combinations of a Phone Number

原文：http://blog.csdn.net/sina012345/article/details/44109069