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LeetCode #143: Reorder List #140

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Merged
WazedKhan merged 2 commits into from
Jul 2, 2026
Merged

LeetCode #143: Reorder List #140

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LeetCode #143: Reorder List
WazedKhan Jul 2, 2026
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LeetCode #143: Reorder List
WazedKhan Jul 2, 2026
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33 changes: 33 additions & 0 deletions LeetCode/medium/reorder_list_143.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,33 @@
from typing import Optional


class ListNode:
def __init__(self, val=0, next=None):
self.val = val
self.next = next


class Solution:
def reorderList(self, head: Optional[ListNode]) -> None:
slow, fast = head, head

while fast and fast.next:
slow = slow.next
fast = fast.next.next
half = slow.next
slow.next = None
Comment on lines +19 to +26

@coderabbitai coderabbitai Bot Jul 2, 2026

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🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Crashes on empty list input.

If head is None, slow and fast both start as None; the while fast and fast.next loop is skipped, leaving slow as None. Line 25 then does slow.next, raising AttributeError: 'NoneType' object has no attribute 'next'.

🐛 Proposed fix 
     def reorderList(self, head: Optional[ListNode]) -> None:
+        if not head or not head.next:
+            return
         slow, fast = head, head
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
def reorderList(self, head: Optional[ListNode]) -> None:
slow, fast = head, head
while fast and fast.next:
slow = slow.next
fast = fast.next.next
half = slow.next
slow.next = None
def reorderList(self, head: Optional[ListNode]) -> None:
if not head or not head.next:
return
slow, fast = head, head
while fast and fast.next:
slow = slow.next
fast = fast.next.next
half = slow.next
slow.next = None
🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@LeetCode/medium/reorder_list_143.py` around lines 19 - 26, The reorderList
method currently assumes head is non-null, which causes a crash when called with
an empty list. Update reorderList in the Solution class to handle the None case
up front before using slow.next, so the fast/slow split logic only runs when
head exists. Keep the existing midpoint logic and next-pointer splitting intact
after the early return.


prev = None
while half:
next_node = half.next
half.next = prev
prev = half
half = next_node

while head and prev:
next_head = head.next
head.next = prev
next_prev = prev.next
prev.next = next_head
head = next_head
prev = next_prev
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