[LeetCode] 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)

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