leetcode——86. 分隔链表

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def partition(self, head, x):
        """
        :type head: ListNode
        :type x: int
        :rtype: ListNode
        """
        if not head:
            return 
        a=ListNode(-1)
        a.next=head
        p=head
        r=p
        q=p.next
        if p.val>=x:
            p=a
        while q:
            if q.val>=x:
                q=q.next
                r=r.next
            else:
                if q==p.next:
                    p=p.next
                    r=r.next
                    q=q.next
                else:
                    r.next=q.next
                    q.next=p.next
                    p.next=q
                    p=q
                    q=r.next
        return a.next