leetCode 6. ZigZag Conversion

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 s, int numRows);

Example 1:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"

Example 2:

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

P     I    N
A   L S  I G
Y A   H R
P     I

//    方法一超过38.49%
    public String convert(String s, int numRows) {
        if (numRows <= 1 || s.length() <= 1) return s;
        char[][] res = new char[numRows][s.length() * (numRows - 1) / (2 * numRows - 2)+1];
        boolean direction = true;//行走方向，默认是向下走
        int row = 0, col = 0;//下一个填的位置
        for (int i = 0; i < s.length(); i++) {
            res[row][col] = s.charAt(i);
            if (row == numRows - 1) {//触底，改变行走方向
                direction = false;
            }
            if (i != 0 && row == 0) {//碰头，改变行走方向
                direction = true;
            }
            if (direction) {// 向下走
                ++row;
            } else {// 向右上走
                --row;
                ++col;
            }
        }
        s=new String("");
        for (int i = 0; i < res.length; i++) {
            for (int j = 0; j < res[0].length; j++) {
                if (res[i][j] != '\0')
                    s += res[i][j];
            }
        }
        return s;
    }

//  方法二：超过30.42%
    String strOfi(String s, int begin, int step) {
        String rs = new String("");

        while (begin < s.length()) {
            rs += s.charAt(begin);
            begin += step;
        }
        return rs;
    }
    public String convert(String s, int numRows) {
        if (numRows <= 1) return s;
        String rs = new String("");
        //第一行
        rs += strOfi(s, 0, 2 * numRows - 2);
        //中间行
        for (int i = 1; i < numRows-1; i++) {
            String s1 = new String("");
            String s2 = new String("");
            s1=strOfi(s,i,2*numRows-2);
            s2=strOfi(s,2*numRows-i-2,2*numRows-2);

            int j,k;
            for (j = 0,k=0; j < s1.length() && k<s2.length(); j++,k++) {
                rs+=s1.charAt(j);
                rs+=s2.charAt(k);
            }
            if (j < s1.length()){
                rs+=s1.charAt(j);
            }
            if (k<s2.length()){
                rs+=s2.charAt(k);
            }
        }
        //最后一行
        rs += strOfi(s, numRows - 1, 2 * numRows - 2);
        return rs;
    }