题目描述:给定一个单链表 L:L0→L1→…→Ln-1→Ln ,将其重新排列后变为: L0→Ln→L1→Ln-1→L2→Ln-2→…
你不能只是单纯的改变节点内部的值,而是需要实际的进行节点交换。
示例 1:
给定链表 1->2->3->4, 重新排列为 1->4->2->3.示例 2:
给定链表 1->2->3->4->5, 重新排列为 1->5->2->4->3.
解法1。方法就是找到链表中间节点,断开成两段,把后半段逆转,然后再两个链表合并,互相改变next指针,如下
# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def reorderList(self, head):
"""
:type head: ListNode
:rtype: void Do not return anything, modify head in-place instead.
"""
if not head or not head.next:
return
# 找到中间节点,切断
slow = head
fast = slow.next
while fast and fast.next:
slow = slow.next
fast = fast.next.next
mid = slow.next
slow.next = None
# 逆转链表
pre = None
cur = mid
while cur:
tmp = cur.next
cur.next = pre
pre = cur
cur = tmp
# 前后两段链表合并
head1 = head
head2 = pre
while head1 and head2:
tmp = head2.next
head2.next = head1.next
head1.next = head2
head1 = head1.next.next
head2 = tmp