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".
Solution: 
所有行的重复周期都是 gap = 2 * nRows - 2

对于首行和末行之间的行，还会额外重复一次，重复的这一次距离本周期起始字符的距离是 gap - 2 * i.
Run time complexity is O(n), constant space.

public class Solution {
    public String convert(String s, int nRows) {
        if(s == null || s.length() == 0 || nRows <= 0) {
            return "";
        }
        if(nRows == 1) {
            return s;
        }
        
        StringBuilder res = new StringBuilder();
        int gap = 2 * nRows - 2;
        for(int i = 0; i < nRows; i++) {
            for(int j = i; j < s.length(); j += gap) {
                res.append(s.charAt(j));
                if(i != 0 && i != nRows - 1 && j + gap - 2*i < s.length()) {
                    res.append(s.charAt(j + gap - 2*i));
                }
            }
        }
        return res.toString();
    }
}