Question

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

string convert(string s, int numRows);

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

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

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

My Answer

class Solution {
     public String convert(String s, int numRows) {
        if (numRows <= 1) {
            return s;
        }

        // 初始化
        StringBuilder[] sb = new StringBuilder[numRows];
        for (int i = 0; i < numRows; i++) {
            sb[i] = new StringBuilder("");
        }

        // 遍历数据，写入sb中
        // true向下走，false向上走
        boolean flag = true;
        int index = 0;
        for (int i = 0; i < s.length(); i++) {
            sb[index].append(s.charAt(i));

            // 从第一行，向下走
            if (index == 0) {
                flag = true;
            }
            // 最后一行，往上走
            if (index == numRows - 1) {
                flag = false;
            }
            
            if (flag) {
                index++;
            } else {
                index--;
            }
        }

        // 组装
        StringBuilder result = new StringBuilder("");
        for (StringBuilder temp : sb) {
            result.append(temp);
        }

        return result.toString();
    }
}