Leetcode48. Rotate Image

題目

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?

思路

先上下顛倒，在對角線顛倒
/*
* clockwise rotate
* first reverse up to down, then swap the symmetry
* 1 2 3 7 8 9 7 4 1
* 4 5 6 => 4 5 6 => 8 5 2
* 7 8 9 1 2 3 9 6 3
*/

測試用力

[]
[1]
[1,2,3
 4,5,6
 7,8,9]

代碼

package leetcodeArray;

public class Leetcode48RotateImage {
    public void rotate(int[][] matrix) {

        int N = matrix.length;

        for(int i = 0; i < N / 2; i++){
            for(int j = 0; j < matrix[i].length; j++){
                int temp = matrix[i][j];
                matrix[i][j] = matrix[N - i - 1][j];
                matrix[N - i - 1][j] = temp;
            }
        }

        for(int i = 0; i < matrix.length; i++){
            for(int j = i; j < matrix[i].length; j++){
                int temp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = temp;
            }
        }
    }
}

結果

他山之玉

/*
 * clockwise rotate
 * first reverse up to down, then swap the symmetry 
 * 1 2 3     7 8 9     7 4 1
 * 4 5 6  => 4 5 6  => 8 5 2
 * 7 8 9     1 2 3     9 6 3
*/
void rotate(vector<vector<int> > &matrix) {
    reverse(matrix.begin(), matrix.end());
    for (int i = 0; i < matrix.size(); ++i) {
        for (int j = i + 1; j < matrix[i].size(); ++j)
            swap(matrix[i][j], matrix[j][i]);
    }
}

/*
 * anticlockwise rotate
 * first reverse left to right, then swap the symmetry
 * 1 2 3     3 2 1     3 6 9
 * 4 5 6  => 6 5 4  => 2 5 8
 * 7 8 9     9 8 7     1 4 7
*/
void anti_rotate(vector<vector<int> > &matrix) {
    for (auto vi : matrix) reverse(vi.begin(), vi.end());
    for (int i = 0; i < matrix.size(); ++i) {
        for (int j = i + 1; j < matrix[i].size(); ++j)
            swap(matrix[i][j], matrix[j][i]);
    }
}

class Solution:
    def rotate(self, A):
        n = len(A)
        for i in range(n/2):
            for j in range(n-n/2):
                A[i][j], A[~j][i], A[~i][~j], A[j][~i] = \
                         A[~j][i], A[~i][~j], A[j][~i], A[i][j]