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Problem solution · C++

Rooted Tree Isomorphism Classification

Rooted Tree Isomorphism Classification: a C++ solution using hash-based lookup. Learn the idea, check the complexity, and read the full code, with credit to NyaanNyaan Competitive Programming Library.

Technique
Hash-based lookup
Source
NyaanNyaan Competitive Programming Library
Length
38 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

Hash-based lookup

For Rooted Tree Isomorphism Classification, the implementation stores previously seen values or frequencies in a hash table for direct membership and lookup operations.

  1. Decide the key that represents the information needed later.
  2. Update its count or stored state while scanning the input.
  3. Use constant-time expected lookups to detect matches or assemble the result.

Code notes

  • 38 lines of C++ from the credited upstream file yosupo-tree-hash.test.cpp.
  • The implementation visibly relies on hash lookup.
  • 2 loop blocks detected.

Complexity

Expected hash operations are constant time, but the surrounding scan and the number of stored keys determine total work and memory.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from NyaanNyaan Competitive Programming Library by NyaanNyaan and is used under the CC0-1.0 licence.

Open problem ↗Original source ↗
Full codeRooted Tree Isomorphism Classification · C++C++
Use this to learn the idea, then write your own version.
#define PROBLEM "https://judge.yosupo.jp/problem/rooted_tree_isomorphism_classification"//#include "../../template/template.hpp"//#include "../../hashmap/hashmap-unerasable.hpp"//#include "../../tree/rooted-tree-hash.hpp"using namespace Nyaan; void q() {  ini(N);  vvi g(N);  for (int i = 1; i < N; i++) {    ini(p);    g[p].push_back(i);  }  RootedTreeHash th{g};  UnerasableHashMap<typename RootedTreeHash<vvi>::Hash, int> mp;  int K = 0;  vi ans(N, -1);   rep(i, N) {    if (mp.count(th.hash[i]) == 0) {      mp[th.hash[i]] = ans[i] = K++;    } else {      ans[i] = mp[th.hash[i]];    }  }  out(K);  rep(i, N) cout << ans[i] << " ";} void Nyaan::solve() {  int t = 1;  // in(t);  while (t--) q();}

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