1363. ZigZag Conversion

public class Solution {
    /**
     * @param s: the given string
     * @param numRows: the number of rows
     * @return: the string read line by line
     */
    public String convert(String s, int numRows) {
        // Write your code here
        if (nRows <= 1) return s;
        string res = "";
        int size = 2 * nRows - 2;
        for (int i = 0; i < nRows; ++i) {
            for (int j = i; j < s.size(); j += size) {
                res += s[j];
                int tmp = j + size - 2 * i;
                if (i != 0 && i != nRows - 1 && tmp < s.size()) res += s[tmp];
            }
        }
        return res;
    }
}