LeetCode – Rotate Image (Java)

http://segmentfault.com/a/1190000003819186

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?

为了Inplace实现这题，我们要用交换的方法，顺序是左上先和左下交换，然后左上和右下交换，然后左上和右上交换。和Spiral Matrix类似，我们通过圈数来控制内外的顺序。

    public void rotate(int[][] matrix) {
        // 计算圈数
        int n = matrix.length, lvl = n / 2;
        for(int i = 0; i < lvl; i++){
            for(int j = i; j < n - i - 1; j++){
                // 左上和左下交换
                swap(matrix, i, j, j, n - i - 1);
                // 左上和右下交换
                swap(matrix, i, j, n - i - 1, n - j - 1);
                // 左上和右上交换
                swap(matrix, i, j, n - j - 1, i);
            }
        }
    }
    
    private void swap(int[][] matrix, int i1, int j1, int i2, int j2){
        int tmp = matrix[i1][j1];
        matrix[i1][j1] = matrix[i2][j2];
        matrix[i2][j2] = tmp;
    }

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; } } }

Read full article from LeetCode – Rotate Image (Java)