题目说是z字形变化，但其实模拟更像n字形变化，找到字符下标规律就逐个拼接就能得到答案

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