[Leetcode 66] 48 Rotate Image

Problem:

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?

 

Analysis:

Please reference the answer on stackoverflow. 

http://stackoverflow.com/questions/3488691/how-to-rotate-a-matrix-90-degrees-without-using-any-extra-space

Transpose and interchange rows or columns (depends whether you want to rotate left or right).

It's a totally in-place version.

 

1) original matrix

1 2 3
4 5 6
7 8 9

2) transpose

1 4 7
2 5 8
3 6 9

3-a) change rows to rotate left

3 6 9
2 5 8
1 4 7

3-b) or change columns to rotate right

7 4 1
8 5 2
9 6 3

Code:

class Solution {
public:
    void rotate(vector<vector<int> > &matrix) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int tmp;
        
        // transpose the matrix
        for (int i=0; i<matrix.size(); i++) {
            for (int j=i+1; j<matrix.size(); j++) {
                tmp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = tmp;
            }
        }
        
        // switch columns
        for (int i=0, j=matrix.size()-1; i<j; i++, j--) {
            for (int k=0; k<matrix.size(); k++) {
                tmp = matrix[k][i]; 
                matrix[k][i] = matrix[k][j];
                matrix[k][j] = tmp;
            }
        }
    }
};