Add solution for 143. Reorder List

Author: sean424

reorder-list/bhyun-kim.py

@@ -0,0 +1,73 @@
+"""
+143. Reorder List
+https://leetcode.com/problems/reorder-list/
+"""
+
+from typing import Optional
+
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+"""
+Solution:
+    To reorder the linked list, we can follow these steps:
+    First, find the middle of the linked list using the slow and fast pointers.
+    Reverse the second half of the linked list.
+    Merge the first half and the reversed second half.
+
+    1. Find the middle of the linked list using the slow and fast pointers
+        - Initialize the slow and fast pointers to the head of the linked list
+        - Move the slow pointer by one step and the fast pointer by two steps 
+          until the fast pointer reaches the end of the linked list.
+    2. Reverse the second half of the linked list
+        - Initialize the prev and curr pointers to None and the middle node, respectively
+        - Iterate through the second half of the linked list
+            - Store the next node of the current node
+            - Reverse the current node
+            - Move the prev and curr pointers to the next nodes
+    3. Merge the first half and the reversed second half
+        - Initialize the first and second pointers to the head and the reversed second half, respectively
+        - Iterate through the linked list
+            - Store the next nodes of the first and second pointers
+            - Update the next pointers of the first and second pointers
+            - Move the first and second pointers to the next nodes
+
+Time complexity: O(N)
+    - We iterate through the linked list to find the middle node and reverse the second half
+    - The time complexity is O(N)
+
+Space complexity: O(1)
+    - We only use constant extra space for the pointers
+    - The space complexity is O(1)
+
+"""
+
+
+class Solution:
+    def reorderList(self, head: Optional[ListNode]) -> None:
+        """
+        Do not return anything, modify head in-place instead.
+        """
+        if not head or not head.next:
+            return
+        
+        slow, fast = head, head
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+        prev, curr = None, slow
+        while curr:
+            next_temp = curr.next
+            curr.next = prev
+            prev = curr
+            curr = next_temp
+
+        first, second = head, prev
+        while second.next:
+            tmp1, tmp2 = first.next, second.next
+            first.next = second
+            second.next = tmp1
+            first, second = tmp1, tmp2