24. Swap Nodes in Pairs {Medium}

24. Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->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.

shufanlin's solution:

# 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
        """
        dummy = ListNode(0)
        dummy.next = head
        
        prev = dummy
        cur = head
        
        while cur and cur.next:
            nxt = cur.next
            prev.next = nxt
            cur.next = nxt.next
            nxt.next = cur
            
            prev = cur
            cur = cur.next
        return dummy.next