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Problem solution · C++

Find Edges in Shortest Paths

Find Edges in Shortest Paths: a C++ 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
56 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 Find Edges in Shortest Paths, 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

  • 56 lines of C++ from the credited upstream file 3123.cpp.
  • The implementation visibly relies on sequence storage, work queue.
  • 4 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 codeFind Edges in Shortest Paths · C++C++
Use this to learn the idea, then write your own version.
class Solution { public:  // Similar to 2203. Minimum Weighted Subgraph With the Required Paths  vector<bool> findAnswer(int n, vector<vector<int>>& edges) {    vector<bool> ans;    vector<vector<pair<int, int>>> graph(n);     for (const vector<int>& edge : edges) {      const int u = edge[0];      const int v = edge[1];      const int w = edge[2];      graph[u].emplace_back(v, w);      graph[v].emplace_back(u, w);    }     const vector<int> from0 = dijkstra(graph, 0);    const vector<int> from1 = dijkstra(graph, n - 1);     for (const vector<int>& edge : edges) {      const int u = edge[0];      const int v = edge[1];      const int w = edge[2];      ans.push_back(from0[u] + w + from1[v] == from0[n - 1] ||                    from0[v] + w + from1[u] == from0[n - 1]);    }     return ans;  }  private:  static constexpr int kMax = 1'000'000'000;   vector<int> dijkstra(const vector<vector<pair<int, int>>>& graph, int src) {    vector<int> dist(graph.size(), kMax);     dist[src] = 0;    using P = pair<int, int>;  // (d, u)    priority_queue<P, vector<P>, greater<>> minHeap;    minHeap.emplace(dist[src], src);     while (!minHeap.empty()) {      const auto [d, u] = minHeap.top();      minHeap.pop();      if (d > dist[u])        continue;      for (const auto& [v, w] : graph[u])        if (d + w < dist[v]) {          dist[v] = d + w;          minHeap.emplace(dist[v], v);        }    }     return dist;  }};

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