class Solution {
public:
string convert(string s, int numRows) {
if (numRows == 1) return s;
string ret;
int n = s.size();
int cycleLen = 2 * numRows - 2;
for (int i = 0; i < numRows; i++) {
for (int j = 0; j + i < n; j += cycleLen) {
ret += s[j + i];
if (i != 0 && i != numRows - 1 && j + cycleLen - i < n)
ret += s[j + cycleLen - i];
}
}
return ret;
}
};