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Problem solution · Python

Subtree Inversion Sum

Subtree Inversion Sum: a Python solution using dynamic programming. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Dynamic programming
Source
walkccc LeetCode Solutions
Length
35 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

Dynamic programming

For Subtree Inversion Sum, the implementation records answers for smaller states and reuses them to build the requested result without repeating work.

  1. Define precisely what one DP state represents.
  2. Establish the base cases before transitions are evaluated.
  3. Process states in dependency order and combine only already-known values.

Code notes

  • 35 lines of Python from the credited upstream file 3544.py.
  • The implementation visibly relies on sequence storage, cached states.
  • No explicit loop blocks detected.

Complexity

Multiply the number of reachable states by the work performed for each transition, then include the stored state table in memory usage.

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 codeSubtree Inversion Sum · PythonPython
Use this to learn the idea, then write your own version.
class Solution:  def subtreeInversionSum(      self,      edges: list[list[int]],      nums: list[int],      k: int  ) -> int:    n = len(edges) + 1    parent = [-1] * n    graph = [[] for _ in range(n)]     for u, v in edges:      graph[u].append(v)      graph[v].append(u)     @functools.lru_cache(None)    def dp(u: int, stepsSinceInversion: int, inverted: bool) -> int:      """      Returns the maximum sum for subtree rooted at u, with      `stepsSinceInversion` steps of inversion and `inverted` is true if the      subtree is inverted.      """      num = -nums[u] if inverted else nums[u]      negNum = -num      for v in graph[u]:        if v == parent[u]:          continue        parent[v] = u        num += dp(v, min(k, stepsSinceInversion + 1), inverted)        if stepsSinceInversion == k:          negNum += dp(v, 1, not inverted)      return max(num, negNum) if stepsSinceInversion == k else num     return dp(0, k, False)

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