Rotate a Matrix

Flip the matrix upside down. Then mirror it along the diagonal where i and j swap places.

void rotate(vector<vector<int>>& mat) {
    reverse(mat.begin(), mat.end());
    for(int i=0;i<mat.size();++i)
        for(int j=i+1;j<mat[0].size();++j)
            swap(mat[i][j], mat[j][i]);
}

This works because rotation equals reversal plus transposition. The first step flips everything vertically. The second step swaps elements across the main diagonal.

The code runs in O(n²) time. It uses constant extra space since all operations happen in place.

#array