单调栈

单调栈

//伪代码
stack<int> st;
for (遍历数组)
{
	if (栈空 || 栈顶元素>=当前元素)
	{
		入栈;
	}
	else
	{
		while (栈非空 && 栈顶元素<当前元素)
		{
			栈顶元素出栈;
			更新结果;
		}
		当前元素入栈;
	}
}

Stack<int> S;
for(int i=1 ;i<=n ;i++){
	while(S.size() && a[S.top()] >= a[i]) S.pop();
	
    if(S.empty())     L[i] = 0;
    else              L[i] = S.top();
    
    S.push(i);
 }

主要借鉴:
总结一
HDU 1506

#include<cstdio>
#include<iostream>
#include<stack>
#include<algorithm>
using namespace std;
typedef long long ll;
ll h[100010];
int l[100010];
int r[100010];
stack<int>s;
ll max(ll a, ll b)
{
    if (a > b)return a;
    else return b;
}
int main()
{
    int n;
    while (cin >> n && n)
    {
        memset(h, 0, sizeof(h));
        memset(l, 0, sizeof(l));
        memset(r, 0, sizeof(r));
        for (int i = 0; i < n; i++)
        {
            cin >> h[i];
        }
        while (s.size())s.pop();
        for (int i = 0; i <n ; i++)
        {
            while (s.size() && h[s.top()] >= h[i])s.pop();
            if (s.empty())l[i] =0;
            else l[i] = s.top()+1;//数组从0开始,push(i),下一对应坐边界应为i+1;
            s.push(i);
        }
        while (s.size())s.pop();
        for (int i = n-1; i >=0; i--)
        {
            while (s.size() && h[s.top()] >= h[i])s.pop();
            if (s.empty())r[i] = n;
            else r[i] = s.top();
            s.push(i);
        }
        ll ans = 0;
        for (int i = 0; i < n; i++) 
        {
            ans = max(ans, h[i] * (r[i] - l[i]));
        }
        cout<<ans<<'\n';
    }
    return 0;
}

总结二