ZigZag Conversion

1. Question

给定行数，将某字符串转换为zigzag形式，然后按行输出。zigzag形式如：

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".

 2. Solution

O(n)，遍历每个字符，依次将其加入第m行的字符串中，最后将所有行的字符串组合起来

public class Solution {
    public String convert( String s, int nRows ){
        if( nRows == 1 )    return s;
        StringBuilder[] res = new StringBuilder[nRows];
        for( int i=0; i<nRows; i++ )
            res[i] = new StringBuilder();
        int loop = 2 * nRows-2;
        int mid = nRows-1;
        for( int i=0; i<s.length(); i++ )
            res[ mid - Math.abs( i % loop - mid ) ].append(s.charAt(i));
        for( int i = 1; i<nRows; i++ )
            res[0].append( res[i] );
        return res[0].toString();
    }
}