Area under histogram

#include <bits/stdc++.h>
#define maxm 1000001
#define ll long long
using namespace std;

ll T, N, x, max_area, area;
stack<ll> st;
vector<ll> hist(maxm);

int main()
{
    ios_base::sync_with_stdio(false);
    cin>>T;
    while(T--){
        cin>>N;
        max_area = 0;
        for(ll i=0; i<N; ++i)
            cin>>hist[i];
        for(ll i=0; i<N; ++i){
            if(st.empty() || hist[st.top()] <= hist[i])
                st.push(i);
            else{
                while(!st.empty() && hist[st.top()] > hist[i]){
                    x = hist[st.top()];
                    st.pop();
                    area = st.empty() ? (x * i) : (x * (i-1-st.top()));
                    max_area = max(max_area, area);
                }
                st.push(i);
            }
        }
        while(!st.empty()){
            x = hist[st.top()];
            st.pop();
            area = st.empty() ? (x * N) : (x * (N-1-st.top()));
            max_area = max(max_area, area);
        }
        cout<<max_area<<endl;
    }
    return 0;
}