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class Solution 
{
public:
    vector<int> spiralOrder(vector<vector<int>>& matrix) 
    {
        if (matrix.empty()) return {};
        vector<int> res;
        int l = 0;                      //左边界
        int r = matrix[0].size() - 1;   //右边界
        int t = 0;                      //上边界
        int b = matrix.size() - 1;      //下边界
        while (true)
        {
            //left -> right
            for (int i = l; i <= r; i++) res.push_back(matrix[t][i]);
            if (++t > b) break;
            //top -> bottom
            for (int i = t; i <= b; i++) res.push_back(matrix[i][r]);
            if (--r < l) break;
            //right -> left
            for (int i = r; i >= l; i--) res.push_back(matrix[b][i]);
            if (--b < t) break;
            //bottom -> top
            for (int i = b; i >= t; i--) res.push_back(matrix[i][l]);
            if (++l > r) break;
        }
        return res;
    }
};