leetcode: 59. Spiral Matrix II

# Given an integer n, 
# generate a square matrix filled with elements from 1 to n2 in spiral order.
#
# For example,
# Given n = 3,
#
# You should return the following matrix:
# [
#     [ 1, 2, 3 ],
#     [ 8, 9, 4 ],
#     [ 7, 6, 5 ]
# ]

class Solution():
    def generateMatrix(self, n):
        x = [[0 for _ in range(n)] for __ in range(n)]
        left, right, top, bottom, num = 0, n - 1, 0, n - 1, 1
        while top <= bottom:
            for i in range(left, right + 1):
                x[top][i], num = num, num+1
            for i in range(top + 1, bottom):
                x[i][right], num = num, num+1
            if top < bottom:
                for i in reversed(range(left, right + 1)):
                    x[bottom][i], num = num, num+1
                for i in reversed(range(top + 1, bottom)):
                    x[i][left], num = num, num+1
            left, right, top, bottom = left + 1, right - 1, top + 1, bottom - 1
        return x


if __name__ == "__main__":
    assert Solution().generateMatrix(3) == [[1, 2, 3], [8, 9, 4], [7, 6, 5]]