6. ZigZag Conversion java solutions

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 text, int nRows);

convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".

这题一上来就蒙，呵呵、、、每次做这种边界index 寻找的题目都很忧伤。。leetcode的 case 太多了。

public class Solution {
    public String convert(String s, int numRows) {
        if(s == null || s.length() == 1 || numRows == 1) return s;
        StringBuilder ans = new StringBuilder();
        int size = 2*numRows - 2;
        for(int i = 0; i < numRows; i++){
            for(int j = i; j < s.length(); j += size){
                ans.append(s.charAt(j));
                if(i != 0 && i != (numRows - 1)){// 不在首行和尾行的情况
                    int tmp = j + size - 2*i;
                    if(tmp < s.length()) ans.append(s.charAt(tmp));// 加上中间的字符
                }
            }
        }
        return ans.toString();
    }
}