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Swap Nodes in Pairs

Question​

Given a linked list, write a function to swap the positions of the nodes in each pair of nodes in the linked list.

Example 1
Input:  1->2->3->4
Output: 2->1->4->3

Solution​

all//Swap Nodes in Pairs.py


# Definition for singly-linked list.
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next


class Solution:
    def swapPairs(self, head):
        if not head or not head.next:
            return head

        dummy = ListNode(0)
        dummy.next = head
        current = dummy
        while current.next and current.next.next:
            first_node = current.next
            second_node = current.next.next
            first_node.next = second_node.next
            current.next = second_node
            current.next.next = first_node
            current = current.next.next
        return dummy.next