ZigZag Conversion

c++

class Solution {public:    string convert(string s, int numRows) {        if (s.empty()) return s;        vector<string> cache(numRows);        for (int i = 0; i < s.size();) {            int idx = 0;            for (idx = 0; idx < numRows && i<s.size(); ++idx,++i) {                cache[idx].push_back(s[i]);            }            idx -= 2;            for (; idx >0 && i<s.size(); --idx, ++i) {                cache[idx].push_back(s[i]);            }        }        string tmp;        for (auto v : cache) {            tmp += v;        }        return tmp;    }};