0059：螺旋矩阵 II（★）

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class Solution:
    def generateMatrix(self, n: int) -> List[List[int]]:
        i,j,di,dj = 0,0,0,1
        res = [[0]*n for _ in range(n)]
        for x in range(1,n*n+1):
            res[i][j] = x
            if res[(i+di)%n][(j+dj)%n]:
                di,dj = dj,-di
            i,j = i+di,j+dj
        return res