LeetCode#6 Z字形变换

class Solution {
public:
    string convert(string s, int numRows) {
        if(numRows==1){
            return s;
        }
        vector<string> list(numRows);
        int i,flag;
        flag=-1;
        i=0;
        for(auto str:s){
            list[i].push_back(str);
            if(i==0||i==numRows-1){
                flag=-1*flag;
            }
            i+=flag;
        }
        string answer;
        for(i=0;i<numRows;i++){
            answer+=list[i];
        }
        return answer;
    }
};