欧拉筛+并查集

集合 - 洛谷

std::vector<int> minp, primes,primes1;void sieve(int n,int p) {minp.assign(n + 1, 0);primes.clear();for (int i = 2; i <= n; i++) {if (minp[i] == 0) {minp[i] = i;primes.push_back(i);}for (auto p : primes) {if (i * p > n) {break;}minp[i * p] = p;if (p == minp[i]) {break;}}}
}
struct DSU {std::vector<int>f, siz;DSU(){}DSU(int n) {init(n);}void init(int n) {f.resize(n);std::iota(f.begin(), f.end(), 0);siz.assign(n, 1);}int find(int x) {while (x != f[x]) {x = f[x] = f[f[x]];}return x;}bool same(int x, int y) {return find(x) == find(y);}bool merge(int x, int y) {x = find(x);y = find(y);if (x == y)return false;siz[x] += siz[y];f[y] = x;return true;}int size(int x) {return siz[find(x)];}
};
int main() {std::ios::sync_with_stdio(false), std::cin.tie(0), std::cout.tie(0);int a, b, p;std::cin >> a >> b >> p;DSU dsu(b+1);sieve(b, p);for (int i = 0; i < primes.size(); i++) {if (primes[i] >= p)primes1.push_back(primes[i]);}for (int i = 0; i < primes1.size(); i++) {int c = 0;while (c * primes1[i] < a)c++;while (primes1[i] * (c + 1) <= b) {dsu.merge(primes1[i] * c, primes1[i] * (c + 1));c++;}}int ans = 0;for (int i = a; i <= b; i++) {if (i == dsu.find(i))ans++;}std::cout << ans << '\n';return 0;
}