From LeetCode 24. Swap Nodes in Pairs


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.)


Input: head = [1,2,3,4]
Output: [2,1,4,3]


class Solution:
    def swapPairs(self, head: ListNode) -> ListNode:
        res = ListNode(next=head)
        pre = res
        # we have to make sure and are not null value, otherwise swap has ended
        while and
            cur =
            post =
            # pre,cur,post corresponds to leftmost, middle and rightmost nodes
   = cur
   = post

            pre =

Credit to yuzhoujr