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Problem solution · Python

Maximum Twin Sum of a Linked List

Maximum Twin Sum of a Linked List: a Python solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Direct simulation
Source
walkccc LeetCode Solutions
Length
30 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Direct simulation

For Maximum Twin Sum of a Linked List, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 30 lines of Python from the credited upstream file 2130.py.
  • The implementation keeps its working state in language-native values and containers.
  • No explicit loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeMaximum Twin Sum of a Linked List · PythonPython
Use this to learn the idea, then write your own version.
class Solution:  def pairSum(self, head: ListNode | None) -> int:    def reverseList(head: ListNode) -> ListNode:      prev = None      while head:        next = head.next        head.next = prev        prev = head        head = next      return prev     ans = 0    slow = head    fast = head     # `slow` points to the start of the second half.    while fast and fast.next:      slow = slow.next      fast = fast.next.next     # `tail` points to the end of the reversed second half.    tail = reverseList(slow)     while tail:      ans = max(ans, head.val + tail.val)      head = head.next      tail = tail.next     return ans

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