Spiral Matrix

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

For example,
Given the following matrix:

[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]

You should return [1,2,3,6,9,8,7,4,5].

class Solution {
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        if(matrix.empty()) return vector<int>();
        
        int m = matrix.size();
        int n = matrix[0].size();

        vector<int> res(m*n,0);

        int x1 = 0; 
        int y1 = 0;
        int x2 = m - 1;
        int y2 = n - 1;

        int num = 0;
        while(x1 <= x2 && y1 <= y2){
            for(int i=y1;i<=y2;i++) res[num++] = matrix[x1][i];
            for(int i=x1+1;i<=x2;i++) res[num++] = matrix[i][y2];
            if(x2 != x1)
                for(int i=y2-1;i>=y1;i--) res[num++] =matrix[x2][i];
            if(y1 != y2)   
                for(int i=x2-1;i>x1;i--) res[num++] = matrix[i][y1];

            x1++,y1++,x2--,y2--;
        } 
        return res;
    }
};