Spiral Matrix

https://leetcode.com/explore/learn/card/array-and-string/202/introduction-to-2d-array/1168/

四个指针，四种情况。其实很简单，背下来。

class Solution:
    def spiralOrder(self, matrix: List[List[int]]) -> List[int]:
        res = []
        m = len(matrix)
        if m == 0:
            return res
        n = len(matrix[0])
        left = 0
        right = n-1
        bottom = 0
        top = m-1
        d = 0
        while left <= right and bottom <= top:
            if d == 0:
                for i in range(left,right+1):
                    res.append(matrix[bottom][i])
        
                bottom+=1
                d=1
            elif d ==1 :
                for i in range(bottom,top+1):
                    res.append(matrix[i][right])
                right-=1
                d=2
            elif d==2 :
                for i in range(right,left-1,-1):
                    res.append(matrix[top][i])
                top-=1
                d=3
            elif d == 3:
                for i in range(top,bottom-1,-1):
                    res.append(matrix[i][left])
                left+=1
                d=0
        return res