LeetCode #17 电话号码的字母组合

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        map<int,string> phone={{2,"abc"},
                                {3,"def"},
                                {4,"ghi"},
                                {5,"jkl"},
                                {6,"mno"},
                                {7,"pqrs"},
                                {8,"tuv"},
                                {9,"wxyz"}};
        vector<string> ans;
        string s;
        int i,m,j,flag;
        vector<int> nums(digits.size());
        if(digits.size()==0){
            return ans;
        }
        for(i=0;i<digits.size();i++){
            nums[i]=digits[i]-'0';
        }
        vector<int> n(digits.size(),0);
        m=phone[nums[0]].size();
        while(n[0]<m){
            s="";
            for(j=0;j<n.size();j++){
                s.push_back(phone[nums[j]][n[j]]);
            }
            ans.push_back(s);
            flag=1;
            j=n.size()-1;
            while(flag&&j>=0){
                n[j]+=flag;
                if(n[j]==phone[nums[j]].size()&&j!=0){
                    n[j]=0;
                    flag=1;
                }
                else{
                    flag=0;
                }
                j--;
            }
        }
        return ans;
    }
};