Submission #1250725

#TimeUsernameProblemLanguageResultExecution timeMemory
1250725thegamercoder193개의 봉우리 (IOI25_triples)C++20
51 / 100
2093 ms100136 KiB
#include <iostream> #include <vector> #include <numeric> #include <map> #include <set> #include <tuple> #include <algorithm> // Placeholder for Part II, required by the grader to compile. std::vector<int> construct_range(int M, int K) { return {}; } // Correctly counts mythical triples without overcounting. long long count_triples(std::vector<int> H) { int N = H.size(); if (N < 3) { return 0; } // Use a set of tuples to store unique mythical triples found. // This elegantly solves the overcounting problem. std::set<std::tuple<int, int, int>> found_triples; // We check all 6 permutations. For each valid triple (i, j, k) found, // we insert it into the set. The set ensures uniqueness. // Iterate through the middle element `j` to structure the search for (int j = 1; j < N - 1; ++j) { // --- Find potential i's (i < j) --- // Case where H[j] = j - i => i = j - H[j] if (H[j] > 0 && j - H[j] >= 0) { int i = j - H[j]; if (i < j) { // Now we have (i, j) and need to find k > j // Case 3: H[i]=b, H[j]=a, H[k]=a+b int k = j + H[i]; if (k > j && k < N && H[k] == H[i] + H[j]) { found_triples.insert({i, j, k}); } // Case 5: H[i]=a+b, H[j]=a, H[k]=b k = i + H[i]; if (k > j && k < N && H[i] > H[j] && H[k] == H[i] - H[j]) { found_triples.insert({i, j, k}); } } } } // --- Forward pass for cases anchored at i --- for (int i = 0; i < N; ++i) { // Case where H[i] = j - i => j = i + H[i] if (H[i] > 0 && i + H[i] < N) { int j = i + H[i]; if (j > i) { // Now we have (i, j) and need to find k > j // Case 1: H[i]=a, H[j]=b, H[k]=a+b if (H[j] > 0) { int k = j + H[j]; if (k > j && k < N && H[k] == H[i] + H[j]) { found_triples.insert({i, j, k}); } } // Case 2: H[i]=a, H[j]=a+b, H[k]=b int k = i + H[j]; if (k > j && k < N && H[j] > H[i] && H[k] == H[j] - H[i]) { found_triples.insert({i, j, k}); } } } } // --- Backward pass for cases anchored at k --- for (int k = N-1; k >= 0; --k) { // Case where H[k] = k - j => j = k - H[k] if(H[k] > 0 && k - H[k] >= 0) { int j = k - H[k]; if (j < k) { // Now we have (j, k) and need to find i < j // Case 6: H[i]=a+b, H[j]=b, H[k]=a int i = k - H[i]; // This is not right, H[i] is unknown. // Let's re-derive: k-i = H[i], so k = i+H[i] // This case is difficult to anchor at k. } } } // A more robust map-based approach for the remaining complex cases // to ensure all are found without overcounting. std::map<int, std::vector<int>> i_plus_Hi; std::map<int, std::vector<int>> k_minus_Hk; for(int i = 0; i < N; ++i) i_plus_Hi[i+H[i]].push_back(i); for(int k = 0; k < N; ++k) k_minus_Hk[k-H[k]].push_back(k); // Case 4: H[i]=b, H[j]=a+b, H[k]=a. => k-i=H[j], j-i=H[k], k-j=H[i] // From this: j = i + H[k] and k = j + H[i] => k-H[i] = i+H[k] for(auto const& [val, i_list] : i_plus_Hi) { if(k_minus_Hk.count(val)) { for(int i : i_list) { for(int k : k_minus_Hk[val]) { if (i < k) { int j = i + H[k]; if (j > i && j < k && H[j] == H[i] + H[k]) { found_triples.insert({i, j, k}); } } } } } } // Case 6: H[i]=a+b, H[j]=b, H[k]=a => k-i=H[i], j-i=H[k], k-j=H[j] // From this: k=i+H[i] and j=i+H[k] // And k-H[j]=j => k=j+H[j] // So we need i+H[i] = j+H[j] = k for(auto const& [k, i_list] : i_plus_Hi) { if (k >= 0 && k < N) { // k must be a valid index for(int i : i_list) { // Now find a `j` such that j+H[j] = k and i < j < k if(i_plus_Hi.count(k)) { for(int j : i_plus_Hi[k]) { if (i < j && j < k) { // Ensure order if (H[i] > H[j] && H[k] == H[i] - H[j]) { found_triples.insert({i, j, k}); } } } } } } } return found_triples.size(); }
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