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Problem solution · Python

Sort List

Sort 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
50 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 Sort 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

  • 50 lines of Python from the credited upstream file 148.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 codeSort List · PythonPython
Use this to learn the idea, then write your own version.
class Solution:  def sortList(self, head: ListNode) -> ListNode:    def split(head: ListNode, k: int) -> ListNode:      while k > 1 and head:        head = head.next        k -= 1      rest = head.next if head else None      if head:        head.next = None      return rest     def merge(l1: ListNode, l2: ListNode) -> tuple:      dummy = ListNode(0)      tail = dummy       while l1 and l2:        if l1.val > l2.val:          l1, l2 = l2, l1        tail.next = l1        l1 = l1.next        tail = tail.next      tail.next = l1 if l1 else l2      while tail.next:        tail = tail.next       return dummy.next, tail     length = 0    curr = head    while curr:      length += 1      curr = curr.next     dummy = ListNode(0, head)     k = 1    while k < length:      curr = dummy.next      tail = dummy      while curr:        l = curr        r = split(l, k)        curr = split(r, k)        mergedHead, mergedTail = merge(l, r)        tail.next = mergedHead        tail = mergedTail      k *= 2     return dummy.next

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