51nod-1102 面积最大的矩形

思路：

单调栈，维护左右边界

#include <iostream>
#include <stdio.h>
#include <algorithm>
#include <math.h>
#include <queue>
#include <stack>
using namespace std;
typedef long long ll;
ll a[50005];
ll l[50005];
ll r[50005];
int main()
{
int n;
cin>>n;
for(int i=1;i<=n;i++)
cin>>a[i];
stack<ll> q;
while(!q.empty())
q.pop();
for(int i=1;i<=n;i++)
{
if(q.empty())
{
l[i]=i;
q.push(i);
}
else
{
l[i]=i;
while(!q.empty()&&a[q.top()]>=a[i] )
{
l[i]=l[q.top()];
q.pop();
}
q.push(i);
}
}
while(!q.empty())
q.pop();
for(int i=n;i>=1;i--)
{
if(q.empty())
{
r[i]=i;
q.push(i);
}
else
{
r[i]=i;
while(!q.empty()&&a[q.top()]>=a[i] )
{
r[i]=r[q.top()];
q.pop();
}
q.push(i);
}
}
long long ans=0;
for(int i=1;i<=n;i++)
{
long long temp=( r[i] - l[i] +1)*a[i];
ans=max(ans,temp);
}
cout<<ans<<endl;
return 0;
}