java n*n的二维数组转置，不开辟新的空间

public class RotateImage {
    public static void rotate(int[][] matrix) {
        if(matrix==null||matrix.length==0)
            return;
        int i,j;
        int len=matrix.length;
        for (i=0;i<len;i++)
        {
            for(j=0;j<len;j++)
            {
                if(i<j) {
                    int temp = matrix[i][j];
                    matrix[i][j] = matrix[j][i];
                    matrix[j][i] = temp;
                }
            }
        }
        for (i=0;i<len;i++)
        {
            for(j=0;j<len;j++)
            {
                System.out.print(matrix[i][j]+" ");
            }
            System.out.println();
        }

    }
    public static void main(String[] args){
        int [][] matrix={{1,2,3},{4,5,6},{7,8,9}};
        RotateImage.rotate(matrix);
    }
}