LeetCode(17) Letter Combinations of a Phone Number

class Solution {
public:
	map<char,string> coll;
	vector<string> vec;
public:
    vector<string> letterCombinations(string digits) {
        if(digits.length()==0)
            return vec;
        f();
        digui(0,digits,"");
        return vec;
    }
    void f()
    {
    	coll.insert(pair<char,string>('2',"abc"));
    	coll.insert(pair<char,string>('3',"def"));
    	coll.insert(pair<char,string>('4',"ghi"));
    	coll.insert(pair<char,string>('5',"jkl"));
    	coll.insert(pair<char,string>('6',"mno"));
    	coll.insert(pair<char,string>('7',"pqrs"));
    	coll.insert(pair<char,string>('8',"tuv"));
    	coll.insert(pair<char,string>('9',"wxyz"));
	}
	void digui(int i,string str,string res)
     {
     	if(i==str.length())
		{
			vec.push_back(res);
			return ;
		}
     	string tem=coll[str[i]];
     	for(int k=0;k<tem.length();++k)
   	    	digui(i+1,str,res+tem[k]);
	 }
};