48. Rotate Image

Problem Statement

Medium

You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise).

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:

Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [[7,4,1],[8,5,2],[9,6,3]]

Example 2:

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

Constraints:

n == matrix.length == matrix[i].length
1 <= n <= 20
-1000 <= matrix[i][j] <= 1000

C++

Source file

class Solution {
public:
    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[0].size(); j++){
                swap(matrix[i][j], matrix[j][i]);
            }
        }
    }
};

Java

Source file

class Solution {
    public void rotate(int[][] matrix) {
        // arr[i][j] -> arr[i][n-1-j]
        int m = matrix.length, n = matrix[0].length;
        for(int i=0; i<m; i++)
            for(int j=i+1; j<n; j++)
                swap(matrix, i, j, j, i);
        for(int i=0; i<m; i++)
            for(int j=0; j<n/2; j++)
                swap(matrix, i, j, i, n-1-j);
    }
    private void swap(int[][] matrix, int i1, int j1, int i2, int j2){
        int temp = matrix[i1][j1];
        matrix[i1][j1] = matrix[i2][j2];
        matrix[i2][j2] = temp;
    }
}