B - Largest Rectangle in a Histogram 题解（单调栈）

题目链接

题目思路

单调栈求出每个矩形可以向左向右延伸的最大长度。

代码

#include<cstdio>
#include<stack>
#include<cstring>
#include<cmath>
#include<iostream>
#include<algorithm>
#define fi first
#define se second
#define debug printf("I am here\n");
using namespace std;
typedef long long ll;
const int maxn=3e6+5,inf=0x3f3f3f3f;
const ll INF=0x3f3f3f3f3f3f3f3f;
int n,a[maxn];
int sta[maxn],l[maxn],r[maxn];
int main(){
    while(scanf("%d",&n)!=-1&&n){
        ll ans=0;
        for(int i=1;i<=n;i++){
            scanf("%d",&a[i]);
        }
        int top=0;
        for(int i=1;i<=n;i++){//枚举右端点
            while(top&&a[sta[top]]>=a[i]){
                top--;
            }
            l[i]=top==0?1:sta[top]+1;
            sta[++top]=i;
        }
        top=0;
        for(int i=n;i>=1;i--){
            while(top&&a[sta[top]]>=a[i]){
                top--;
            }
            r[i]=top==0?n:sta[top]-1;
            sta[++top]=i;
        }
        for(int i=1;i<=n;i++){
            ans=max(ans,1ll*(r[i]-l[i]+1)*a[i]);
        }
        printf("%lld\n",ans);
    }
    return 0;
}