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?
方阵顺时针旋转90度,方法:
- 按行reverse
- 交换元素 A[i, j] = A[j, i]
逆时针90度方法:
- 按列reverse
for (auto vi : matrix) reverse(vi.begin(), vi.end());
- 交换元素 A[i, j] = A[j, i]
代码们:
// clockwise rotate
// 1. 按行reverse 2. 交换元素 A[i, j] = A[j, i]
/*
* 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>>& A) {
const int n = A.size();
reverse(A.begin(), A.end());
for (int i = 0; i < n; i++) {
for (int j = i + 1; j < n; j++) {
swap(A[i][j], A[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>>& A) {
const int n = A.size();
for(atuo vi : A) reverse(vi.begin(), vi.end());
for (int i = 0; i < n; i++) {
for (int j = i + 1; j < n; j++) {
swap(A[i][j], A[j][i]);
}
}
}