bzoj1588 [HNOI2002]营业额统计——双向链表

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
typedef long long ll;
int const maxn=33000;
int n,pre[maxn],nxt[maxn],rk[maxn];
ll a[maxn],inf=1e12,ans;
bool cmp(int x,int y){return a[x]<a[y];}
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)scanf("%lld",&a[i]),rk[i]=i;
    sort(rk+1,rk+n+1,cmp);
    for(int i=1;i<=n;i++)
    {
        pre[rk[i]]=rk[i-1];
        nxt[rk[i]]=rk[i+1];
    }
    for(int i=n;i>1;i--)
    {
        ll l=inf,r=inf;
        if(pre[i])l=abs(a[i]-a[pre[i]]);
        if(nxt[i])r=abs(a[i]-a[nxt[i]]);
        ans+=min(l,r);
        nxt[pre[i]]=nxt[i];
        pre[nxt[i]]=pre[i];
    }
    ans+=a[1];
    printf("%lld",ans);
    return 0;
}