feat(528-swapping-nodes-in-a-ll): add py3 optimal solution

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

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+# 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

notes/linked_lists.md

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+---
+category: ds
+title: Linked Lists
+---
+
+# Linked Lists
+
+## Finding nth node from beginning and nth node from end
+
+Use two pointer approach to avoid extra space. Initialize the `kth_node_from_end` pointer when the `tmp` pointer reaches `kth` node and keep incrementing from there on.
+
+### Related problems
+
+- https://leetcode.com/problems/swapping-nodes-in-a-linked-list