24_Swap_Nodes_in_Pairs.py

"""
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
"""

# 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: Optional[ListNode]) -> Optional[ListNode]:
        # fake node to save the starting point
        fake_node = ListNode(next=head)
        
        # instantiate 
        prev_node = fake_node
        current_node = head

        while current_node and current_node.next:
            # save the pointers
            next_node = current_node.next
            nextpair = current_node.next.next

            # swap pointers
            next_node.next = current_node
            current_node.next = nextpair
            prev_node.next = next_node

            # increment
            prev_node = current_node
            current_node = nextpair
        return fake_node.next