class Solution {
public:
    vector<int> printMatrix(vector<vector<int> > matrix) {
        //为啥d改成2就不能通过了呢？明明是先往左边走的。。。
        vector<int> res;
        if(matrix.empty()) return res;
        int n = matrix.size(),m = matrix[0].size();
        vector<vector<bool>> st(n,vector(m,false));

        int dx[4] = {-1,0,1,0},dy[4] = {0,1,0,-1};
        int x = 0,y = 0,d = 1;

        for(int i = 0;i < m * n;++i){
            res.push_back(matrix[x][y]);
            st[x][y] = true;
            int a =x + dx[d],b = y + dy[d];
            if(a < 0 || a >= n || b < 0 || b >= m || st[a][b]){
                d = (d + 1) % 4;
                a =x + dx[d],b = y + dy[d];
            }
            x = a,y = b;
        }
        return res;
    }
};