Method reorderList

leetcode/143. Reorder List.py:19–24

(self, head)

class Solution(object):
    def reorderList(self, head):
        if not head or not head.next:
            return
        ahead, behind = self.split(head)
        behind = self.reverse(behind)
        head = self.reConnect(ahead, behind)
    # split the linkedlist in middle
    def split(self, head):
        fast = head