class Solution {

public:

    vector<int> printMatrix(vector<vector<int> > matrix) {

        int row = matrix.size();

        int col = matrix[].size();

        vector<int> res;

        // 输入的二维数组非法，返回空的数组

        if (row ==  || col == )  return res;

        //定义四个关键变量，表示左上和右下的打印范围

        int left = , top = , right = col - , bottom = row - ;

        while (left <= right && top <= bottom){

            // left to right

            for (int i = left; i <= right; ++i)

                res.push_back(matrix[top][i]);

            // top to bottom

            for (int i = top + ; i <= bottom; ++i)

                res.push_back(matrix[i][right]);

            if (top != bottom)

                for (int i = right - ; i >= left; --i)

                    res.push_back(matrix[bottom][i]);

            if (left != right)

                for (int i = bottom - ; i > top; --i)

                    res.push_back(matrix[i][left]);

            left++,top++,right--,bottom--;

        }

        return res;

    }

};