feat(work): implement reorder list solution for problem 143

work/1/Medium/Linked List/143.reorder-list.py

@@ -18,7 +18,7 @@ L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 →
 You may not modify the values in the list's nodes. Only nodes
 themselves may be changed.
 
- 
+
 
 Example 1:
 
@@ -30,15 +30,16 @@ Example 2:
 Input: head = [1,2,3,4,5]
 Output: [1,5,2,4,3]
 
- 
+
 
 Constraints:
 
-	• The number of nodes in the list is in the range [1, 5 * 10^4].
+        • The number of nodes in the list is in the range [1, 5 * 10^4].
 
-	• 1 <= Node.val <= 1000
+        • 1 <= Node.val <= 1000
 """
 
+
 # Definition for singly-linked list.
 # class ListNode:
 #     def __init__(self, val=0, next=None):
@@ -46,7 +47,25 @@ Constraints:
 #         self.next = next
 class Solution:
     def reorderList(self, head: ListNode | None) -> None:
-        """
-        Do not return anything, modify head in-place instead.
-        """
-        
+        # Find middle
+        slow, fast = head, head.next
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+        # Reverse second half
+        second = slow.next
+        prev = slow.next = None
+        while second:
+            tmp = second.next
+            second.next = prev
+            prev = second
+            second = tmp
+
+        # merge two halfs
+        first, second = head, prev
+        while second:
+            tmp1, tmp2 = first.next, second.next
+            first.next = second
+            second.next = tmp1
+            first, second = tmp1, tmp2