動機
接ptr
Problem
Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list's nodes (i.e., only nodes themselves may be changed.)
Example 1:
Input: head = [1,2,3,4]Output: [2,1,4,3]
Example 2:
Input: head = []Output: []
Example 3:
Input: head = [1]Output: [1]
Constraints:
- The number of nodes in the list is in the range
[0, 100]
. 0 <= Node.val <= 100
Sol
class Solution:
def swapPairs(self, head: ListNode) -> ListNode:
if not head or not head.next:
return head
else:
a,prev = head, None
ret = head.next
while a and a.next:
b,nexta = a.next, a.next.next
b.next = a
b.next.next = nexta
if prev:
prev.next = b
a,prev = nexta, a
if a:
a.next = None
return ret