leetcode/0528_swapping-nodes-in-a-linked-list/python3/solution.py

# Time: O(N)
# Space: O(1)
class Solution:
    def swapNodes(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
        # We are using two pointer approach here
        kth_node = kth_end_node = None
        
        n = 1
        ptr = head
        while ptr != None:
            if n == k:
                kth_node = ptr

            # `kth_end_node` ptr will only start moving once
            # we've traversed k nodes from the beginning of the
            # linked list
            if n >= k:
                kth_end_node = head if kth_end_node is None else kth_end_node.next
            
            n += 1
            ptr = ptr.next
        
        kth_node.val, kth_end_node.val = kth_end_node.val, kth_node.val
        
        return head