I introduced this problem before: https://pekempey.hatenablog.com/entry/2018/09/18/085432
This technique can be applied to shakutori (also known as two pointers or sliding window). Its implementation is a little complicated but it might be efficient.
#include <iostream> #include <vector> #include <algorithm> #include <cassert> using namespace std; int gcd(int x, int y) { return y == 0 ? x : gcd(y, x % y); } int f(vector<int> a) { const int n = a.size(); int res = 0; for (int i = 0; i < n; i++) { int g = 0; for (int j = i; j < n; j++) { g = gcd(g, a[j]); if (g != 1) { res = max(res, j - i + 1); } } } return res; } int g(vector<int> a) { const int n = a.size(); vector<pair<int, int>> L, R; L.emplace_back(0, 0); R.emplace_back(0, 0); int j = 0; int res = 0; for (int i = 0; i < n; i++) { while (j < n && gcd(a[j], gcd(L.back().first, R.back().first)) != 1) { L.emplace_back(gcd(L.back().first, a[j]), a[j]); j++; } res = max(res, j - i); if (i == j) j++; else { if (R.size() == 1) { while (L.size() >= 2) { R.emplace_back(gcd(R.back().first, L.back().second), L.back().second); L.pop_back(); } } R.pop_back(); } } return res; } int main() { assert(f({1,2,1,2,6,1}) == 2); assert(f({1,10,1}) == 1); assert(f({1,1,1}) == 0); assert(g({1,2,1,2,6,1}) == 2); assert(g({1,10,1}) == 1); assert(g({1,1,1}) == 0); }
This technique is described in [1] : http://hirzels.com/martin/papers/debs17-tutorial.pdf
[1] M. Hirzel, S. Schneider, K. Tangwongsan. Sliding-Window Aggregation Algorithms: Tutorial, DEBS'17 Proceedings of the 11th ACM International Conference on Distributed and Event-based Systems, pp.11--14, 2017.