ZigZag

6. ZigZag Conversion

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

P   A   H   N
A P L S I I G
Y   I   R

And then read line by line: "PAHNAPLSIIGYIR" Write the code that will take a string and make this conversion given a number of rows:
string convert(string text, int nRows);
convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".

class Solution {
public:
    string convert(string s, int numRows) {
        size_t len = s.size();
        if (len <= 1 || numRows <= 1) return s;

        vector<string> row(numRows);

        int i = 0;
        int j = len;

        while (i < j) {
            for (int k = 0; k < numRows && i < j; ++k) {
                row[k] += s[i++];
            }

            for (int k = numRows - 2; k > 0 && i < j; --k) {
                row[k] += s[i++];
            }
        }

        string res;
        for (auto str : row) res += str;

        return res;
    }
};