cpp

#include <vector>

// 1st: 5 10 15 20
// 2nd: 1
// 2nd: 5 7
// 2nd: 5 10
// 2nd: 5 12
// 2nd: 7

// lower_bound(x) returns: >= x
// upper_bound(x) return > x
// equal_range(): [returns: >= x; return > x]


void Helper(std::vector<int> &      Result,
            const std::vector<int> &First,
            const std::vector<int> &Second,
            int &                   fi,
            int &                   si) {
   if(fi >= First.size())
      return;
   const auto Pair            = std::equal_range(Second.begin() + si, Second.end(), First[fi]);
   const auto FirstLargerOrEq = Pair.first;
   const auto FirstLarger     = Pair.second;
   if(FirstLargerOrEq == Second.end())
      return;
   Result.push_back(*FirstLargerOrEq);
   si = FirstLarger - Second.begin();   // std::distance



   //   for(; si != Second.size() && Second[si] <= First[fi]; ++si)
   //      if(Second[si] == First[fi])
   //         if(Result.empty() || Result.back() != Second[si])
   //            Result.push_back(Second[si]);
}
std::vector<int> EqualElems(const std::vector<int> &First, const std::vector<int> &Second) {
   std::vector<int> Result;
   for(int fi = 0, si = 0; fi != First.size() && si != Second.size();) {
      Helper(Result, First, Second, fi, si);
      Helper(Result, Second, First, si, fi);
   }
   return Result;
}