Educational Codeforces Round 23 D. Imbalanced Array(单调栈)

 1 #include<bits/stdc++.h>
 2 #define F(i,a,b) for(int i=a;i<=b;++i)
 3 using namespace std;
 4 typedef long long ll;
 5 typedef pair<int,int> P;
 6 
 7 const int N=1e6+7;
 8 int n,a[N],l[N],r[N],top;
 9 ll ans;
10 P S[N];
11 void work()
12 {
13     top=1,S[1]=P(N,0);
14     F(i,1,n)
15     {
16         while(S[top].first<=a[i])top--;
17         l[i]=S[top].second+1;
18         S[++top]=P(a[i],i);
19     }
20     top=1,S[1]=P(N,n+1);
21     for(int i=n;i;i--)
22     {
23         while(S[top].first<a[i])top--;
24         r[i]=S[top].second-1;
25         S[++top]=P(a[i],i);
26     }
27     F(i,1,n)ans+=1ll*a[i]*(i-l[i]+1)*(r[i]-i+1);
28 }
29 
30 int main(){
31     scanf("%d",&n);
32     F(i,1,n)scanf("%d",a+i);
33     work();
34     F(i,1,n)a[i]*=-1;
35     work();
36     printf("%lld\n",ans);
37     return 0;
38 }