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tags: data_structure_python
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# Swap Nodes in Pairs <img src="https://img.shields.io/badge/-medium-orange">

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

You may not modify the values in the list's nodes, only nodes itself may be changed.

**Example:**

# Solution

# 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: ListNode) -> ListNode:
if head == None or head.next == None:
return head
else:
# 1) Swap adjacents nodes.
tmp = head.next
head.next = head.next.next
tmp.next = head
head = tmp
# 2) Attach nodes.
head.next.next = self.swapPairs(head.next.next)
return head