顺时针打印矩阵

输入一个矩阵，按照从外向里以顺时针的顺序依次打印出每一个数字。

样例

输入：
[
[1, 2, 3, 4],
[5, 6, 7, 8],
[9,10,11,12]
]

输出：[1,2,3,4,8,12,11,10,9,5,6,7]

class Solution {
public:
    vector<int> printMatrix(vector<vector<int> > matrix) {
        vector<int> res;
        int n = matrix.size();
        if(!n) return res;
        int m = matrix[0].size();
        
        
        vector<vector<bool>> st(n, vector<bool>(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 < n*m; 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;
    }
};