LeetCode – Rotate Image (Java)

You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Follow up:
Could you do this in-place?

Naive Solution

In the following solution, a new 2-dimension array is created to store the rotated matrix, and the result is assigned to the matrix at the end. This is WRONG! Why?

public class Solution {
    public void rotate(int[][] matrix) {
        if(matrix == null || matrix.length==0)
            return ;
 
        int m = matrix.length;
 
        int[][] result = new int[m][m];
 
        for(int i=0; i<m; i++){
            for(int j=0; j<m; j++){
                result[j][m-1-i] = matrix[i][j];
            }
        } 
 
        matrix = result;
    }
}

The problem is that Java is pass by value not by refrence! “matrix” is just a reference to a 2-dimension array. If “matrix” is assigned to a new 2-dimension array in the method, the original array does not change. Therefore, there should be another loop to assign each element to the array referenced by “matrix”. Check out “ Java pass by value .”

public class Solution {
    public void rotate(int[][] matrix) {
        if(matrix == null || matrix.length==0)
            return ;
 
        int m = matrix.length;
 
        int[][] result = new int[m][m];
 
        for(int i=0; i<m; i++){
            for(int j=0; j<m; j++){
                result[j][m-1-i] = matrix[i][j];
            }
        } 
 
       for(int i=0; i<m; i++){
            for(int j=0; j<m; j++){
                matrix[i][j] = result[i][j];
            }
        } 
    }
}

In-place Solution

By using the relation “matrix[i][j] = matrix[n-1-j][i]“, we can loop through the matrix.

public void rotate(int[][] matrix) {
	int n = matrix.length;
	for (int i = 0; i < n / 2; i++) {
		for (int j = 0; j < Math.ceil(((double) n) / 2.); j++) {
			int temp = matrix[i][j];
			matrix[i][j] = matrix[n-1-j][i];
			matrix[n-1-j][i] = matrix[n-1-i][n-1-j];
			matrix[n-1-i][n-1-j] = matrix[j][n-1-i];
			matrix[j][n-1-i] = temp;
		}
	}
}