LeetCode

Rotate Image

2013.12.15 21:40

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?

Solution:

　　Rotate an nXn 2D matrix in-place. See the picture below and that's how I did the rotation:

　　　　ABCDA

　　　　DEFEB

　　　　CFXFC

　　　　BEFED

　　　　ADCBA

　　Time complexity is O(n^2), space complexity is O(1).

Accepted code:

// 1AC, yes!!!
class Solution {
public:
    void rotate(vector<vector<int> > &matrix) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int n;
        
        n = matrix.size();
        
        if(n <= 0){
            return;
        }
        
        int i, j, tmp;
        int leni = n / 2;
        int lenj = n - leni;
        
        for(i = 0; i < leni; ++i){
            for(j = 0; j < lenj; ++j){
                // extra O(1)
                tmp = matrix[i][j];
                matrix[i][j] = matrix[n - 1 - j][i];
                matrix[n - 1 - j][i] = matrix[n - 1 - i][n - 1 - j];
                matrix[n - 1 - i][n - 1 - j] = matrix[j][n - 1 - i];
                matrix[j][n - 1 - i] = tmp;
            }
        }
    }
};