86.分割链表

给定一个链表和一个特定值 x，对链表进行分隔，使得所有小于 x 的节点都在大于或等于 x 的节点之前。

你应当保留两个分区中每个节点的初始相对位置。

示例：

输入: head = 1->4->3->2->5->2, x = 3
输出: 1->2->2->4->3->5

思路：用两个链表分别存小于x和大于等于x的元素，然后连接两个链表。

# 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):
        f, b = ListNode(-1), ListNode(-1)
        f1, b1 = f, b
        while head:
            if head.val < x:
                f1.next, head = head, head.next
                f1 = f1.next
                f1.next = None
            else:
                b1.next, head = head, head.next
                b1 = b1.next
                b1.next = None
        f1.next = b.next
        return f.next

#自己写的代码如下：
class Solution(object):
    def partition(self, head, x):
        dummynode = ListNode(None)
        dummynode.next = None
        pre = dummynode
        p = head
        cur = p
        while cur!=None:
            if cur.val<x:
                if p == cur:
                    cur = cur.next
                else:
                    p.next = cur
                    p = cur
                    cur = cur.next
            else:
                pre.next = cur
                pre = cur
                cur = cur.next
        pre.next = None
        if dummynode.next != None:
            p.next = dummynode.next
        return head