48. Rotate Image

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?

例如

1	2	3
4	5	6
7	8	9

变成

7	4	1
8	5	2
9	6	3

观察规律，新阵(i,j)=原阵(j,n-1-i)

public class Solution {
public void rotate(int[][] matrix)
{
int row=matrix.length;
if(row<1)
return ;
int[][] arr=new int[row][row];

for(int i=0;i<row;i++)
System.arraycopy(matrix[i], 0, arr[i], 0, row);

for(int i=0;i<row;i++)
for(int j=0;j<row;j++)
matrix[j][row-1-i]=arr[i][j];

}
}
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in-place方法，摘自https://discuss.leetcode.com/topic/6796/a-common-method-to-rotate-the-image

here give a common method to solve the image rotation problems.

/*
* 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]);
}
}