Spiral Matrix

class Solution {
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) {
        int m = matrix.size(), n = m ? matrix[0].size() : 0, u = 0, d = m - 1, l = 0, r = n - 1, p = 0;
        vector<int> order(m * n);
        while (u <= d && l <= r) {
            for (int col = l; col <= r; col++) {
                order[p++] = matrix[u][col];
            }
            if (++u > d) {
                break;
            }
            for (int row = u; row <= d; row++) {
                order[p++] = matrix[row][r];
            }
            if (--r < l) {
                break;
            }
            for (int col = r; col >= l; col--) {
                order[p++] = matrix[d][col];
            }
            if (--d < u) {
                break;
            }
            for (int row = d; row >= u; row--) {
                order[p++] = matrix[row][l];
            }
            if (l++ > r) {
                break;
            }
        }
        return order;
    }
};