54. 螺旋矩阵

https://leetcode.cn/problems/spiral-matrix/description/
给你一个 m 行 n 列的矩阵 matrix ，请按照 顺时针螺旋顺序 ，返回矩阵中的所有元素。

class Solution {

    private List<int[]> vector = Arrays.asList(
        new int[]{0,1},
        new int[]{1, 0},
        new int[]{0,-1},
        new int[]{-1, 0}
    );

    private boolean over(int x, int y, int m, int n) {
        return x < 0 || y < 0 || x >= m || y>= n;
    }

    public List<Integer> spiralOrder(int[][] matrix) {
        int d = 0;
        int m = matrix.length;
        int n = matrix[0].length;
        int x = 0;
        int y = -1;
        int xx; int yy;
        boolean[][] visit = new boolean[m][n];
        List<Integer> res = new ArrayList<>();
        while (true) {
            xx = x + vector.get(d % 4)[0];
            yy = y + vector.get(d % 4)[1];
            if (over(xx, yy,m,n) || visit[xx][yy]) {
                d += 1;
                continue;
            }
            res.add(matrix[xx][yy]);
            visit[xx][yy] = true;
            x = xx; y = yy;
            if (res.size() >= m *n){
                break;
            }
        }
        return res;
    }
}