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Problem solution · Java

Checking Existence of Edge Length Limited Paths II

Checking Existence of Edge Length Limited Paths II: a Java solution using disjoint set union. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Disjoint set union
Source
walkccc LeetCode Solutions
Length
54 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

Disjoint set union

For Checking Existence of Edge Length Limited Paths II, the implementation maintains connected components and merges them as relationships are processed.

  1. Give each element a component representative.
  2. Merge representatives when a connection is accepted.
  3. Answer connectivity or component queries from the compressed representatives.

Code notes

  • 54 lines of Java from the credited upstream file 1724.java.
  • The implementation visibly relies on sequence storage, ordered lookup.
  • 2 loop blocks detected.

Complexity

Account for every find and union operation; with path compression and ranked merging, the amortized cost is nearly constant per operation.

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 codeChecking Existence of Edge Length Limited Paths II · JavaJava
Use this to learn the idea, then write your own version.
class UnionFind {  public UnionFind(int n) {    id = new TreeMap[n];    for (int i = 0; i < n; ++i) {      id[i] = new TreeMap<>();      id[i].put(0, i);    }  }   public void union(int u, int v, int limit) {    final int i = find(u, limit);    final int j = find(v, limit);    if (i == j)      return;    id[i].put(limit, j);  }   public int find(int u, int limit) {    // the maximum key of id[u] <= limit    final int floorKey = id[u].floorKey(limit);    final int i = id[u].get(floorKey);    if (i == u)      return u;    // Recursively find i's id.    final int j = find(i, limit);    id[u].put(limit, j);    return j;  }   // id[i]'s (key, value) := (limit, id of node i <= limit)  private TreeMap<Integer, Integer>[] id;} class DistanceLimitedPathsExist {  public DistanceLimitedPathsExist(int n, int[][] edgeList) {    uf = new UnionFind(n);     Arrays.sort(edgeList, Comparator.comparingInt(edge -> edge[2]));     for (int[] edge : edgeList) {      final int u = edge[0];      final int v = edge[1];      final int d = edge[2];      uf.union(u, v, d);    }  }   public boolean query(int p, int q, int limit) {    return uf.find(p, limit - 1) == uf.find(q, limit - 1);  }   private UnionFind uf;}

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