min_element,max_elementの使い方が分からない（C++)

Question

AtcoderBeginnerContestのC問題(https://atcoder.jp/contests/abc189/tasks/abc189_c)を解いていてmin_elementを使いたいと思いコードを書いたのですが、良く分からないエラーが出ました。多分原因はmin_elementだと思いますが、どう直したら良いのか分からないのでご教授願います。

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define rep(i,a,n) for (ll i = a;i<(n);++i)
#define all(v) v.begin(),v.end()
#define bit(n) (1LL<<(n))
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
class range {private: struct I{int x;int operator*(){return x;}bool operator!=(I& lhs){return x<lhs.x;}void operator++(){++x;}};I i,n;
public:range(int n):i({0}),n({n}){}range(int i,int n):i({i}),n({n}){}I& begin(){return i;}I& end(){return n;}};
/*------------------------------------------------------------------------------------------------------------------------------------------------*/
int main() {
    ll n;
    cin>>n;
    vector<ll>a(n);
    rep(i,0,n)cin>>a.at(i);
    ll ans=0;
    rep(i,0,n){
        rep(j,i,n){
            ll min=min_element(a.at(i),a.at(j));
            ans=max(ans,min*(j-i+1));
        }
    }
    cout<<ans;
	return 0;
}

エラー内容

In file included from /usr/include/c++/6/bits/stl_algobase.h:71:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/istream:38,
                 from /usr/include/c++/6/sstream:38,
                 from /usr/include/c++/6/complex:45,
                 from /usr/include/c++/6/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:52,
                 from prog.cpp:1:
/usr/include/c++/6/bits/predefined_ops.h: In instantiation of ‘constexpr bool __gnu_cxx::__ops::_Iter_less_iter::operator()(_Iterator1, _Iterator2) const [with _Iterator1 = long long int; _Iterator2 = long long int]’:
/usr/include/c++/6/bits/stl_algo.h:5460:12:   required from ‘constexpr _ForwardIterator std::__min_element(_ForwardIterator, _ForwardIterator, _Compare) [with _ForwardIterator = long long int; _Compare = __gnu_cxx::__ops::_Iter_less_iter]’
/usr/include/c++/6/bits/stl_algo.h:5484:43:   required from ‘constexpr _FIter std::min_element(_FIter, _FIter) [with _FIter = long long int]’
prog.cpp:21:47:   required from here
/usr/include/c++/6/bits/predefined_ops.h:43:16: error: invalid type argument of unary ‘*’ (have ‘long long int’)
       { return *__it1 < *__it2; }
                ^~~~~~
/usr/include/c++/6/bits/predefined_ops.h:43:25: error: invalid type argument of unary ‘*’ (have ‘long long int’)
       { return *__it1 < *__it2; }
                         ^~~~~~

Answer 1

min_element() に渡すのはイテレータ、返るのもイテレータです。

ll min = *min_element(a.begin() + i, a.begin() + j);

でしょうか。

Answer 2

min_elementの戻り値をlong longで受けているからではないですかね

min_elementはiteratorを返すと思います

std::vector<long long> a = {100,30,30,4};

int main() {
	decltype(a)::iterator min = std::min_element(a.begin(),a.end());//ok
    //long long min = std::min_element(a.begin(),a.end());//error
	
	std::cout << *min;//4
}