每日一题: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".

解答

public String convert(String s, int nRows){
        if(nRows == 1){
            return s;
        }

        StringBuffer sb = new StringBuffer();
        int step = 2 * nRows - 2;
        int length = s.length();

        for(int i = 0; i < length; i += step){
            sb.append(s.charAt(i));
        }

        for(int i = 1; i  < nRows - 1; i++){
            for(int  j = i; j < length; j+= step){
                sb.append(s.charAt(j));
                if(j + step - 2 * i < length){
                    sb.append(s.charAt(j + step - 2 * i));
                }
            }
        }

        for(int i = nRows - 1; i < length; i += step){
            sb.append(s.charAt(i));
        }

        return sb.toString();
    }

注解

一般性的找规律题。

按照生成的顺序，发现首行和末行都是以2 * nRows - 2增加的。

中间nRow - 2行，在2 * nRow - 2步数增加的情况下，加入2 * nRows - 2 - 2 * (nRow - 2)小步数增加。