Rotate Image
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.
Constraints:
- n == matrix.length == matrix[i].length
- 1 <= n <= 20
- -1000 <= matrix[i][j] <= 1000
Examples:
Input: [[1,2,3],[4,5,6],[7,8,9]]
Output: [[7,4,1],[8,5,2],[9,6,3]]
Explanation: The image is rotated by 90 degrees clockwise.
Solutions
Transpose and Reverse
Time: O(n^2)Space: O(1)
The solution first transposes the matrix (i.e., swaps the row and column indices of each element), and then reverses each row to achieve the rotation effect.
void rotate(vector<vector<int>>& matrix) {
int n = matrix.size();
for (int i = 0;
i < n;
i++) {
for (int j = i;
j < n;
j++) {
swap(matrix[j][i], matrix[i][j]);
}
}
for (int i = 0;
i < n;
i++) {
reverse(matrix[i].begin(), matrix[i].end());
}
}Follow-up:
How would you rotate the image by 180 degrees or 270 degrees?
