Letter Combinations of a Phone Number

遍历所有的可能。

class Solution {
    private:
    vector<string> res;
    map<char,string> num;
public:
   void initial()
   {
       
        num['2']="abc";
        num['3']="def";
        num['4']="ghi";
        num['5']="jkl";
        num['6']="mno";
        num['7']="pqrs";
        num['8']="tuv";
        num['9']="wxyz";  
   }
      
    void dfsdigits(int layer,int depth,string digits,string ans)
    {
        if(layer==depth)
       {
           res.push_back(ans);
           return ;
       }
       for(int i=0;i<num[digits[layer]].size();i++)
       {
           dfsdigits(layer+1,depth,digits,ans+num[digits[layer]][i]);
       }
    }
    vector<string> letterCombinations(string digits) {
        if(digits.empty()) return res;
        initial();
       
       dfsdigits(0,digits.size(),digits,"");
       return res;
    }
    
};

下面这种是循环，设中间变量

class Solution {
public:
   
    vector<string> letterCombinations(string digits) {
       vector<string> res;
       string num[10]={"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
       if(digits.empty()) return res;
       res.push_back("");
       for(int i=0;i<digits.size();i++)
       {
           vector<string> temp;
           for(int j=0;j<num[digits[i]-'0'].size();j++)
             for(int k=0;k<res.size();k++)
             temp.push_back(res[k]+num[digits[i]-'0'][j]);
             res=temp;
       }
         
         
       return res;
    }
    
};