RotateImage

Suddenly I found some tricks with matrix rotate.
There are two basic operations: transpose() and swapRows().
transpose(): swap matrix[i][j] with matrix[j][i]
swapRows(): swap the first row with the last row and keep swapping until we reach middle;

Rotate Left(counter-clockwise):

1. transpose()
2. swapRows()

Rotate Right(clockwise):

1. swapRows()
2. transpose()

/**
 * Created by RuiG on 11/18/16.
 */
public class RotateImage {
    // 根据flag决定是向左还是向右转
    public static void main(String[] args) {
        int[][] matrix = {{1,2,3}, {4,5,6}, {7,8,9}};
        int[][] ans = null;

        System.out.println("original");
        print(matrix);

//        System.out.println("left");
//        ans = rotateImage(matrix, 0);
//        print(ans);

        System.out.println("right");
        ans = rotateImage(matrix, 1);
        print(ans);
    }

    public static void print(int[][] matrix) {
        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j < matrix[0].length; j++) {
                System.out.print(matrix[i][j] + " ");
            }
            System.out.println();
        }
    }
    // flag == 0, rotate left; flag == 1, rotate right
    public static int[][] rotateImage(int[][] matrix, int flag) {
        if (flag == 0) { // rotate left
            transpose(matrix);
            swapRows(matrix);
        } else { // rotate right
            swapRows(matrix);
            transpose(matrix);
        }
        return matrix;
    }

    public static void swapRows(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0] == null || matrix[0].length == 0) {
            return;
        }
        int m = matrix.length, start = 0, end = m - 1;
        while (start < end) {
            int[] tmp = matrix[start];
            matrix[start] = matrix[end];
            matrix[end] = tmp;

            start++;
            end--;
        }
    }


    // transpose a matrix [i, j] to [j, i]
    public static void transpose(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0] == null || matrix[0].length == 0) {
            return;
        }
        int m = matrix.length, n = matrix[0].length;

        for (int i = 0; i < m; i++) {
            for (int j = i + 1; j < n; j++) {
                int tmp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = tmp;
            }
        }
    }
}