o(n) time and o(1) space using two pointers.

1721. Swapping Nodes in a Linked List/1721. Swapping Nodes in a Linked List.py

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+"""
+You are given the head of a linked list, and an integer k.
+
+Return the head of the linked list after swapping the values of the kth node from the beginning and the kth node from the end (the list is 1-indexed).
+
+
+
+Example 1:
+
+
+Input: head = [1,2,3,4,5], k = 2
+Output: [1,4,3,2,5]
+Example 2:
+
+Input: head = [7,9,6,6,7,8,3,0,9,5], k = 5
+Output: [7,9,6,6,8,7,3,0,9,5]
+
+
+Constraints:
+
+The number of nodes in the list is n.
+1 <= k <= n <= 105
+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 swapNodes(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
+        left,right = head,head
+        for _ in range(k-1):
+            right = right.next
+        r = right
+        while right.next:
+            left = left.next
+            right = right.next
+        r.val,left.val = left.val,r.val
+        return head