POJ3250 Bad Hair Day（单调栈）

#include<cstdio>
#include<cstring>
using namespace std;
#define INF (1<<30)
#define MAXN 88888
int a[MAXN],r[MAXN],stack[MAXN],top;
int main(){
int n;
scanf("%d",&n);
for(int i=1; i<=n; ++i) scanf("%d",a+i);
a[++n]=INF;
for(int i=1; i<=n; ++i){
while(top && a[stack[top]]<=a[i]){
r[stack[top]]=i-1;
--top;
}
stack[++top]=i;
}
long long res=0;
for(int i=1; i<n; ++i) res+=r[i]-i;
printf("%lld",res);
return 0;
}