C++ :

#include<iostream>
#include<cstdio>
using namespace std;
#define MAXN 400000
#define LL long long
int h[400010];
int n;
int Queue[400010];
int L[400010],R[400010];
int main()
{
	cin>>n;
	int i;
	for (i=1;i<=n;i++) cin>>h[i];
	h[0]=h[n+1]=-1;
	Queue[0]=0;
	int Head=0,Tail=1;
	for (i=1;i<=n;i++) {
		while (Head<Tail && h[i]<=h[Queue[Tail-1]]) Tail--;
		L[i]=i-Queue[Tail-1]-1;
		Queue[Tail++]=i;
	}
	Queue[0]=n+1;
	Head=0,Tail=1;
	for (i=n;i>=1;i--) {
		while (Head<Tail && h[i]<=h[Queue[Tail-1]]) Tail--;
		R[i]=Queue[Tail-1]-i-1;
		Queue[Tail++]=i;
	}
	long long MaxArea=0;
	for (i=1;i<=n;i++) {
		long long Area=(L[i]+R[i]+1)*h[i];
		if (Area>MaxArea) MaxArea=Area;
	}
	cout<<MaxArea<<endl;
	return 0;
}

Java :

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Main test = new Main();
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[n];
        for (int i = 0; i < n; i++) arr[i] = sc.nextInt();
        System.out.println(test.largestArea(arr));
    }

    public int largestArea(int[] height){
        if(height == null || height.length == 0) return 0;
        return largestArea(height, height.length);
    }

    private int largestArea(int[] height, int n) {
        int[]left = new int[n];
        int[]right = new int[n];
        left[0] = 0;
        right[n-1] = n-1;

        for (int i = 1; i < n; i++) {
            // 找到最左边的比它大的元素
            int last = i;
            while (last >= 1 && height[i] <= height[last-1]) last = left[last-1];
            left[i] = last;
        }

        for (int i = n-2; i >= 0; i--) {
            // 找到最右边的比它大的元素
            int last = i;
            while (last < n - 1 && height[i] <= height[last+1]) last = right[last+1];
            right[i] = last;
        }

        int ans = 0;
        for (int i = 0; i < n; i++) ans = Math.max(ans,(right[i]-left[i]+1)*height[i]);

        return ans;
    }

}