# 54. Spiral Matrix
###### tags: `spiral`
This the best for spiral!! ref: https://leetcode.com/problems/spiral-matrix/discuss/1466413/Python-simulate-process-explained

class Solution:
    def spiralOrder(self, matrix):
        if not matrix or not matrix[0]:
            return None
        m = len(matrix)
        n = len(matrix[0])
        
        i, j, di, dj = 0, 0, 0, 1
        
        seen = set()
        ans = []
        for _ in range(m*n):
            good = 0<=i+di<m and 0<=j+dj<n and (i+di,j+dj) not in seen
            if not good:
                di, dj = dj, -di  # rotating clockwise
                # right  0,  1
                # down   1,  0
                # left   0, -1
                # up    -1,  0
            ans.append(matrix[i][j])
            seen.add((i,j))
            i, j = i+di, j+dj
        return ans