std::set做法

#include <iostream>
#include <set>
using namespace std;

set<int> operator&(const set<int>& set1, const set<int>& set2) {
    set<int> result;
    auto it1 = set1.begin();
    auto it2 = set2.begin();
    while (it1 != set1.end() && it2 != set2.end()) {
        if (*it1 == *it2) {
            result.insert(*it1);
            ++it1;
            ++it2;
        } else if (*it1 < *it2) {
            ++it1;
        } else {
            ++it2;
        }
    }
    
    return result;
}

int main() {
    set<int>red,blue;
    int start,end,begin,final;
    cin>>start>>end>>begin>>final;
    for (int i=start;i<end;i++) red.insert(i);
    for (int i=begin;i<final;i++) blue.insert(i);
    set<int>result=red & blue;
    cout<<result.size();
    return 0;
}