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)

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 s, int numRows);

Example 1:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"

Example 2:

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

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

1
2                     n-1+n-1
3                 n-1+n-2
*              *
*           *
n-1    n-1+2
n-1+1

将2*(numRows - 1)个元素视为一个循环，每个循环中首行和尾行只有一个元素，特殊处理。

对于中间行 i (由0开始) 的两个元素，左右两个元素的下标差距为2*(numRows - 1 - i)；

Runtime: 3 ms, faster than 97.85%

class Solution {
    public String convert(String s, int numRows) {
        if (numRows == 1 ){
            return s;
        }
        int length = s.length();
        int loop = 2*(numRows - 1);
        StringBuilder res = new StringBuilder();
        for (int i = 0; i < numRows;i++){
            for(int index = i;index < length;index += loop){
                res.append(s.charAt(index));
                int index2 = index + 2*(numRows - 1 - i);
                if (i > 0 && i < numRows - 1 && index2 < length){
                    res.append(s.charAt(index2));
                }
            }
        }
        return res.toString();
    }
}