6. ZigZag Conversion

题目：

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".

链接：https://leetcode.com/problems/zigzag-conversion/#/description

4/4/2017

64ms, 50%

注意的问题：

1. 第3行的判断，需要包括空字符串，输出为1

2. 第6行，sb中每一个StringBuilder都需要初始化

3. 第15行同第3行，需要判断是否是空字符串 --〉更正，这是不需要的，去掉这个判断，运行时间减少25%

public class Solution {
    public String convert(String s, int numRows) {
        if (s.length() == 0 || s == null || numRows == 1) return s;
        StringBuilder[] sb = new StringBuilder[numRows];
        int base = numRows + (numRows - 2);
        for (int i = 0; i < sb.length; i++) sb[i] = new StringBuilder();

        for (int i = 0; i < s.length(); i++) {
            int r = i % base;
            if (r > base / 2) r = base - r;
            sb[r].append(s.charAt(i));
        }
        StringBuilder ret = new StringBuilder();
        for (int i = 0; i < sb.length; i++) {
            ret.append(sb[i]);
        }
        return ret.toString();
    }
}

更多讨论：https://discuss.leetcode.com/category/14/zigzag-conversion