Simple in-place java solution with explanation

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    1. For any element in the array, after the rotation, a[i][j]-->a[j][n-1-i], a[j][n-1-i]-->a[n-1-i][n-1-j], a[n-1-i][n-1-j]-->a[n-1-j][i], a[n-1-j][i]-->a[i][j]
    2. So, every such four elements form a group. Hence we can do the rotation by dealing with total N*N/4 groups.
    3. In the loop, "i<(l+1)/2" and "j<l/2" can also be written as "i<l/2" and "j<(l+1)/2". The purpose is to make sure the N*N/4 groups exactly cover the whole array, no matter the size of array is odd or even.
    public void rotate(int[][] matrix) {
            int l=matrix.length;
            for(int i=0;i<(l+1)/2;i++){//this two lines can make if the size of the array is odd, loop contains
                for(int j=0;j<l/2;j++){//(size-1)/4 steps, if the size is even, loop contains size/4 steps.
                    int p=matrix[i][j];

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    @chuck_long Your current post is much better, but I think the explanation part can be further improved. What do you say?

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    @LHearen Thank you for your advice! I have rewritten the explanation.

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