54. Spiral Matrix

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