LeetCode #6 ZigZag Conversion (E)

[Problem]

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".

[Analysis]

这题比较简单，大概有两类思路：一是找到规律按一定的间隔直接将字符存入buffer，二是分开每行分别建立字符串最后将所有字符串连接起来。前者规律要稍微复杂一点，但内存方面稍微优化。一开始我采用的是将例子写在纸上找规律的暴力方法，只要将规律列清楚，一样可以求解。

[Solution]

public class Solution {
    public String convert(String s, int nRows) {
        if (s.length() <= nRows || nRows == 1) {
            return s;
        }        

        StringBuffer buf = new StringBuffer("");

        for (int i = 0; i < nRows; i++) {
            int interval1 = (nRows - i - 1) * 2;
            int interval2 = i * 2;
            if (interval2 == 0 || interval1 == interval2) {
                for (int j = i; j < s.length(); j += interval1) {
                    buf.append(s.charAt(j));
                }
            } else if (interval1 == 0) {
                for (int j = i; j < s.length(); j += interval2) {
                    buf.append(s.charAt(j));
                }
            } else {
                buf.append(s.charAt(i));
                for (int j = i + interval1; j < s.length(); j += interval1) {
                    buf.append(s.charAt(j));
                    j += interval2;
                    if (j < s.length()) {
                        buf.append(s.charAt(j));
                    }
                }
            }
        }

        return buf.toString();
    }
}