Solve in Python:
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)



Credits:Special thanks to @dietpepsi for adding this problem and creating all test cases.
class Solution:
     def isSelfCrossing(self, x):
         """
         :type x: List[int]
         :rtype: bool
         """
         l = (len(x))
         iscross = False
         if l < 4: return False
         for i in range(3, l):
             #情况1
             if x[i-3]>=x[i-1] and x[i-2]<=x[i]:
                 return True
             #情况2
             if i>=4 and x[i-4]+x[i]>=x[i-2] and x[i-3]==x[i-1]:
                 return True
             #情况3 
             if i>=5 and x[i-5]+x[i-1]>=x[i-3] and x[i-4]+x[i]>=x[i-2] and x[i-2]>=x[i-4] and x[i-2]>x[i-4] and x[i-3]>x[i-5] and x[i-1]<x[i-3]:
                 return True
             iscross = False
         return iscross