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Problem solution · Python

Reachable Nodes In Subdivided Graph

Reachable Nodes In Subdivided Graph: a Python solution using heap or priority queue. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Heap or priority queue
Source
walkccc LeetCode Solutions
Length
51 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

Heap or priority queue

For Reachable Nodes In Subdivided Graph, the implementation repeatedly takes the currently best candidate from a heap while inserting newly available choices.

  1. Define the priority key and whether the smallest or largest item should lead.
  2. Push each candidate when it becomes eligible.
  3. Discard stale entries when necessary and process the best live candidate.

Code notes

  • 51 lines of Python from the credited upstream file 882.py.
  • The implementation visibly relies on sequence storage, work queue.
  • No explicit loop blocks detected.

Complexity

Count heap pushes and pops; each normally contributes a logarithmic factor in the heap size.

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 codeReachable Nodes In Subdivided Graph · PythonPython
Use this to learn the idea, then write your own version.
class Solution:  def reachableNodes(      self,      edges: list[list[int]],      maxMoves: int,      n: int,  ) -> int:    graph = [[] for _ in range(n)]    dist = [maxMoves + 1] * n     for u, v, cnt in edges:      graph[u].append((v, cnt))      graph[v].append((u, cnt))     reachableNodes = self._dijkstra(graph, 0, maxMoves, dist)    reachableSubnodes = 0     for u, v, cnt in edges:      # the number of reachable nodes of (u, v) from `u`      a = 0 if dist[u] > maxMoves else min(maxMoves - dist[u], cnt)      # the number of reachable nodes of (u, v) from `v`      b = 0 if dist[v] > maxMoves else min(maxMoves - dist[v], cnt)      reachableSubnodes += min(a + b, cnt)     return reachableNodes + reachableSubnodes   def _dijkstra(      self,      graph: list[list[tuple[int, int]]],      src: int,      maxMoves: int,      dist: list[int],  ) -> int:    dist[src] = 0    minHeap = [(dist[src], src)]  # (d, u)     while minHeap:      d, u = heapq.heappop(minHeap)      # Already took `maxMoves` to reach `u`, so can't explore anymore.      if dist[u] >= maxMoves:        break      if d > dist[u]:        continue      for v, w in graph[u]:        newDist = d + w + 1        if newDist < dist[v]:          dist[v] = newDist          heapq.heappush(minHeap, (newDist, v))     return sum(d <= maxMoves for d in dist)

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