Leetcode-Rotate Image

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?

Have you met this question in a real interview?

 

Analysis:

We can rotate the outest circle of the matrix, and then move to the next inner circle. I used a recursive method.

Solution:

public class Solution {
    public void rotate(int[][] matrix) {
        int len = matrix.length;
        rotateRecur(matrix,0,0,len);
        
    }

    public void rotateRecur(int[][] matrix, int curX, int curY, int len){
        if (len==1 || len==0) return;

        for (int i=0;i<len-1;i++){
            int temp = matrix[curX][curY+i];
            matrix[curX][curY+i] = matrix[curX+len-1-i][curY];
            matrix[curX+len-1-i][curY] = matrix[curX+len-1][curY+len-1-i];
            matrix[curX+len-1][curY+len-1-i] = matrix[curX+i][curY+len-1];
            matrix[curX+i][curY+len-1] = temp;
        }
        rotateRecur(matrix,curX+1,curY+1,len-2);
    }
            
}

Solution2:

We can easily write the method into iterative method.

public class Solution {
    public void rotate(int[][] matrix) {
        int mLen = matrix.length;
        
        for (int j=0;j<mLen/2;j++){
            int curX = j;
            int curY = j;
            int len = mLen-j*2;
            
            for (int i=0;i<len-1;i++){
                int temp = matrix[curX][curY+i];
                matrix[curX][curY+i] = matrix[curX+len-1-i][curY];
                matrix[curX+len-1-i][curY] = matrix[curX+len-1][curY+len-1-i];
                matrix[curX+len-1][curY+len-1-i] = matrix[curX+i][curY+len-1];
                matrix[curX+i][curY+len-1] = temp;
            }
            
        }
        
    }       
}