873.cpp

class Solution {
  // Runtime: 224 ms, faster than 38.19% of C++ online submissions for Length of
  // Longest Fibonacci Subsequence. Memory Usage: 62.6 MB, less than 7.11% of
  // C++ online submissions for Length of Longest Fibonacci Subsequence.
public:
  int lenLongestFibSubseq(vector<int> &A) {
    unordered_map<int, int> val2idx;
    for (int i = 2; i < A.size(); ++i)
      val2idx[A[i]] = i;
    vector<vector<int>> dp(A.size(), vector<int>(A.size(), 0));
    vector<pair<int, int>> st;
    int ret = 0;
    for (int i = 0; i < A.size(); ++i) {
      for (int j = i + 1; j < A.size(); ++j) {
        for (int a = i, b = j;;) {
          if (auto it = val2idx.find(A[a] + A[b]);
              it != end(val2idx) and dp[a][b] == 0) {
            st.emplace_back(a, b);
            a = b;
            b = it->second;
          } else
            break;
        }
        while (st.size()) {
          const auto [a, b] = st.back();
          st.pop_back();
          dp[a][b] = dp[b][val2idx[A[a] + A[b]]] == 0
                         ? 3
                         : dp[b][val2idx[A[a] + A[b]]] + 1;
          ret = max(ret, dp[a][b]);
        }
      }
    }
    return ret;
  }
};