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

难度：medium

题目：
字符串"PAYPALISHIRING"以之字形表法。

思路：
数组索引遍历下加上减。

Runtime: 22 ms, faster than 80.40% of Java online submissions for ZigZag Conversion.

class Solution {
    public String convert(String s, int numRows) {
        if (1 == numRows) {
            return s;
        }
        
        int idx = 0, signFlg = 1;
        StringBuilder[] sbArray = new StringBuilder[numRows];
        for (int i = 0; i < numRows; i++) {
            sbArray[i] = new StringBuilder();
        }
        
        for (int i = 0; i < s.length(); i++) {
            if (0 == idx) {
                signFlg = 1;
            }
            
            sbArray[idx].append(s.charAt(i));
            idx = (idx + signFlg) % numRows;
            
            if ((numRows - 1) == idx) {
                signFlg = -1;
            }
        }
        
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < numRows; i++) {
            sb.append(sbArray[i]);
        }
        
        return sb.toString();
    }
}