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