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Problem solution · C++

Shortest Path with Alternating Colors

Shortest Path with Alternating Colors: a C++ solution using breadth-first search. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Breadth-first search
Source
walkccc LeetCode Solutions
Length
39 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

Breadth-first search

For Shortest Path with Alternating Colors, the implementation explores reachable states in layers, which is the standard shape for unweighted shortest paths and minimum-step transitions.

  1. Model each valid configuration as a state and each legal move as an edge.
  2. Seed the queue with the starting state and mark it immediately.
  3. Expand each state once, recording distance or reachability for unseen neighbours.

Code notes

  • 39 lines of C++ from the credited upstream file 1129.cpp.
  • The implementation visibly relies on sequence storage, work queue.
  • 5 loop blocks detected.

Complexity

Verify that each state and transition is processed only a bounded number of times; that determines the traversal cost.

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 codeShortest Path with Alternating Colors · C++C++
Use this to learn the idea, then write your own version.
enum class Color { kInit, kRed, kBlue }; class Solution { public:  vector<int> shortestAlternatingPaths(int n, vector<vector<int>>& redEdges,                                       vector<vector<int>>& blueEdges) {    vector<int> ans(n, -1);    vector<vector<pair<int, Color>>> graph(n);  // graph[u] := [(v, edgeColor)]    queue<pair<int, Color>> q{{{0, Color::kInit}}};  // [(u, prevColor)]     for (const vector<int>& edge : redEdges) {      const int u = edge[0];      const int v = edge[1];      graph[u].emplace_back(v, Color::kRed);    }     for (const vector<int>& edge : blueEdges) {      const int u = edge[0];      const int v = edge[1];      graph[u].emplace_back(v, Color::kBlue);    }     for (int step = 0; !q.empty(); ++step)      for (int sz = q.size(); sz > 0; --sz) {        const auto [u, prevColor] = q.front();        q.pop();        ans[u] = ans[u] == -1 ? step : ans[u];        for (auto& [v, edgeColor] : graph[u]) {          if (v == -1 || edgeColor == prevColor)            continue;          q.emplace(v, edgeColor);          v = -1;  // Mark (u, v) as used.        }      }     return ans;  }};

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