【贪心】P1792 [国家集训队]种树 & P1484 种树

将每一个点放进一个优先级队列中，每次弹出一个元素后，可以将它两侧的元素加和减去这个元素本身，再将这个元素放进优先级队列中，我们就能实现了对一次选择的反悔，这样贪心的正确性就有了保证

 

 

P1792代码

#include<bits/stdc++.h>
using namespace std;
const int maxn=2e5+5;
int n,k;
int vis[maxn];
struct point
{
	long long l,r,v;
}a[maxn];
struct node
{
	int val,id;
	bool operator <(node other) const
	{
		return val<other.val;
	}
};
priority_queue <node> q;
long long ans;
int main()
{
//	freopen("P1792_1.in","r",stdin);
//	freopen("a.out","w",stdout);
	scanf("%d%d",&n,&k);
	if(n<k*2)
	{
		printf("Error!");
		return 0;	
	} 
	for(int i=1;i<=n;i++)
	{
		scanf("%lld",&a[i].v);
		a[i].l=i-1; a[i].r=i+1;
		q.push((node){a[i].v,i});
	}
	a[1].l=n; a[n].r=1;
	for(int i=1;i<=k;i++)
	{
		while(vis[q.top().id])
			q.pop();
		node tp=q.top();
		q.pop();
		ans+=tp.val;
		vis[a[tp.id].l]=vis[a[tp.id].r]=1;
		a[tp.id].v=a[a[tp.id].l].v+a[a[tp.id].r].v-a[tp.id].v;
		q.push((node){a[tp.id].v,tp.id});
		a[tp.id].l=a[a[tp.id].l].l;
		a[tp.id].r=a[a[tp.id].r].r;
		a[a[tp.id].l].r=tp.id; a[a[tp.id].r].l=tp.id;
	}
	printf("%lld",ans);
	return 0;
}

 

P1484代码

#include<bits/stdc++.h>
using namespace std;
const int maxn=5e5+5;
int n,k;
int vis[maxn];
struct point
{
	long long l,r,v;
}a[maxn];
struct node
{
	int val,id;
	bool operator <(node other) const
	{
		return val<other.val;
	}
};
priority_queue <node> q;
long long ans;
int main()
{
//	freopen("a.in","r",stdin);
//	freopen("a.out","w",stdout);
	scanf("%d%d",&n,&k);
	for(int i=1;i<=n;i++)
	{
		scanf("%lld",&a[i].v);
		a[i].l=i-1; a[i].r=i+1;
		q.push((node){a[i].v,i});
	}
	for(int i=1;i<=k;i++)
	{
		while(vis[q.top().id])
			q.pop();
		node tp=q.top();
		q.pop();
		if(tp.val<=0)
			break;
		ans+=tp.val;
		vis[a[tp.id].l]=vis[a[tp.id].r]=1;
		a[tp.id].v=a[a[tp.id].l].v+a[a[tp.id].r].v-a[tp.id].v;
		q.push((node){a[tp.id].v,tp.id});
		a[tp.id].l=a[a[tp.id].l].l;
		a[tp.id].r=a[a[tp.id].r].r;
		a[a[tp.id].l].r=tp.id; a[a[tp.id].r].l=tp.id;
	}
	printf("%lld",ans);
	return 0;
}

 

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