335. Self Crossing

一道数学题，考虑一条边被cross的两种情况，然后依次顺延边。

class Solution(object):
    def isSelfCrossing(self, x):
        """
        :type x: List[int]
        :rtype: bool
        """
        b = c = d = e = 0
        for a in x:
            if d >= b > 0 and (a >= c or (a >= c-e >= 0 and f >= d-b)):
                return True
            b, c, d, e, f = a, b, c, d, e
        return False