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24. Swap Nodes in Pairs (Medium)
Given a linked list, swap every two adjacent nodes and return its head.
Example:
Given1->2->3->4
, you should return the list as2->1->4->3
.Note:
- Your algorithm should use only constant extra space.
- You may not modify the values in the list's nodes, only nodes itself may be changed.
蠢办法用n来标记有没有一对,
n = 1时移动到一对中的第一个,记录一下pre
n = 2时 要开始swap了,注意有3个next要变,一个是node.next = pre,pre.next = node.next, ppre.next = node可以画图理解一下,ppre是这一对之前的node,pre是这一对里第一个node,node是这一对里第二个node
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution:
def swapPairs(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
n = 1
if not head:return []
if not head.next:return head
else:cphead = head.next
node = head
ppre = None
while(node):
if n == 1:
pre = node
node = node.next
n = 2
elif n == 2:
aft = node.next
node.next = pre
pre.next = aft
if ppre != None:ppre.next = node
n = 1
ppre = pre
node = aft
return cphead