[LeetCode] 48. Rotate Image

Problem

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

Rotate the image by 90 degrees (clockwise).

Note:

You have to rotate the image in-place, which means you have to modify the input 2D matrix directly. DO NOT allocate another 2D matrix and do the rotation.

Example 1:

Given input matrix =
[
[1,2,3],
[4,5,6],
[7,8,9]
],

rotate the input matrix in-place such that it becomes:
[
[7,4,1],
[8,5,2],
[9,6,3]
]
Example 2:

Given input matrix =
[
[ 5, 1, 9,11],
[ 2, 4, 8,10],
[13, 3, 6, 7],
[15,14,12,16]
],

rotate the input matrix in-place such that it becomes:
[
[15,13, 2, 5],
[14, 3, 4, 1],
[12, 6, 8, 9],
[16, 7,10,11]
]

Solution

Transpose + Flip horizontally

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

4-way swap

class Solution {
    public void rotate(int[][] matrix) {
        int n = matrix.length;
        for (int i = 0; i < n/2; i++) {
            for (int j = i; j < n-i-1; j++) {
                swap(matrix, n, i, j);
            }
        }
    }
    private void swap(int[][] matrix, int n, int i, int 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;
    }
}