[51nod1102]面积最大的矩形(单调栈||预处理)

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
stack<int>s;
ll a[50002];
int main(){
    int n;
    cin>>n;
    for(int i=0;i<n;i++) cin>>a[i];a[n]=-1;
    ll ans=0,tmp;
    for(int i=0;i<=n;i++){
        if(s.empty()||a[i]>a[s.top()]) s.push(i);
        else if(a[i]<a[s.top()]){
            while(!s.empty()&&a[i]<a[s.top()]){
                tmp=s.top();
                s.pop();
                ans=max(ans,1ll*(i-tmp)*a[tmp]);
            }
            s.push(tmp);
            a[tmp]=a[i];
        }
    }
    cout<<ans<<"\n";
    return 0;
} 

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll a[50002],l[50002],r[50002];
int main(){
    int n;
    cin>>n;
    for(int i=1;i<=n;i++) cin>>a[i];
    for(int i=1;i<=n;i++){
        int k=i-1;
        while(a[k]>=a[i]){
            k=l[k]-1;
        }
        l[i]=k+1;
    }
    for(int i=n;i>=1;i--){
        int k=i+1;
        while(a[k]>=a[i]){
            k=r[k]+1;
        }
        r[i]=k-1;
    }
    ll ans=0;
    for(int i=1;i<=n;i++){
        ans=max(ans,(r[i]-l[i]+1)*a[i]);
    }
    cout<<ans<<"\n";
    return 0;
}