Self Crossing

You are given an array x of n positive numbers. You start at point (0,0) and moves x[0] metres to the north, then x[1] metres to the west, x[2] metres to the south, x[3] metres to the east and so on. In other words, after each move your direction changes counter-clockwise.

Write a one-pass algorithm with O(1) extra space to determine, if your path crosses itself, or not.

Example 1: Given x = [2, 1, 1, 2], ┌───┐ │ │ └───┼──> │

Return true (self crossing) Example 2: Given x = [1, 2, 3, 4], ┌──────┐ │ │ │ │ └────────────>

Return false (not self crossing) Example 3: Given x = [1, 1, 1, 1], ┌───┐ │ │ └───┼>

Return true (self crossing)

URL: https://leetcode.com/problems/self-crossing/

class Solution(object):
    def isSelfCrossing(self, x):
        """
        :type x: List[int]
        :rtype: bool
        """
        if x == None or len(x) <= 3:
            return False
        else:
            for i in range(3, len(x)):
                if (x[i-3] >= x[i-1]) and (x[i-2] <= x[i]):
                    return True
                if (i >= 4) and (x[i-4] + x[i] >= x[i-2]) and (x[i-3] == x[i-1]):
                    return True
                if (i>=5) and (x[i-5] <= x[i-3]) and (x[i-4] <= x[i-2]) and (x[i-1] <= x[i-3]) and (x[i-1] >= x[i-3] - x[i-5]) and (x[i] >= x[i-2] - x[i-4]) and (x[i] <= x[i-2]):
                    return True

            return False