HDU 1506 Largest Rectangle in a Histogram

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;

const int maxn=100000+10;
long long a[maxn];
int L[maxn],R[maxn];
int n;

int main()
{
    while(~scanf("%d",&n))
    {
        if(n==0) break;
        for(int i=1;i<=n;i++) scanf("%lld",&a[i]);

        for(int i=1;i<=n;i++) L[i]=i;
        for(int i=2;i<=n;i++)
        {
            if(a[i]>a[i-1]) continue;
            int pre=L[i-1];
            while(1)
            {
                L[i]=pre;
                if(pre==1||a[pre-1]<a[i]) break;
                pre=L[pre-1];
            }
        }

        for(int i=n;i>=1;i--) R[i]=i;
        for(int i=n-1;i>=1;i--)
        {
            if(a[i]>a[i+1]) continue;
            int pre=R[i+1];
            while(1)
            {
                R[i]=pre;
                if(pre==n||a[pre+1]<a[i]) break;
                pre=R[pre+1];
            }
        }

        long long ans=-1;
        for(int i=1;i<=n;i++)
           ans=max(ans,a[i]*(R[i]-L[i]+1));

        printf("%lld\n",ans);
    }
    return 0;
}