浙大17年校赛(ZOJ 3953) Intervals[贪心]

题意:给了n个区间,要求你删去最少的区间,使任意三个区间 a,b,c 不存在 a与b相交,b与c相交,c与a相交 的情况。


分析:比赛时候看到这题,还以为要用什么高深的数据结构,出来听他们说贪心后马上就想到了思路,,还是太菜了。。首先,我们先将n个区间按x从小到大,再y从小到大排序,然后遍历,如果三个区间满足 上述的情况,那么我们将y最大的删去,y相等将x最小的删去。过程中维护两个区间tmp1,tmp2,表示前两个相交的区间是什么,然后就是一些分类,具体看代码。

以下是代码。

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
using namespace std;

#define ull unsigned long long
#define ll long long
#define lson l,mid,id<<1
#define rson mid+1,r,id<<1|1

typedef pairpii;
typedef pairpll;
typedef pairpdd;
const double eps = 1e-6;
const int MAXN = 100005;
const int MAXM = 5005;
const ll LINF = 0x3f3f3f3f3f3f3f3f;
const int INF = 0x3f3f3f3f;
const double FINF = 1e18;
const ll MOD = 1000000007;

struct lx 
{
	lx(){}
	lx(int _x, int _y, int _id)
	{
		x = _x, y = _y, id = _id;
	}
	int x, y, id;
	bool operator<(const lx &a)const 
	{
		if (x == a.x)return y < a.y;
		else return x < a.x;
	}
}tmp[50005];

lx getid(lx a, lx b, lx c)
{
	int id, x, y;
	if (a.y > b.y)
	{
		y = a.y;
		x = a.x;
		id = a.id;
	}
	else if (a.y == b.y)
	{
		if (a.x < b.x)
		{
			y = a.y; x = a.x;
			id = a.id;
		}
		else
		{
			y = b.y; x = b.x;
			id = b.id;
		}
	}
	else
	{
		y = b.y; x = b.x; id = b.id;
	}
	if (c.y > y)x = c.x, y = c.y, id = c.id;
	else if (c.y == y)
	{
		if (c.x < x)
		{
			x = c.x, y = c.y, id = c.id;
		}
	}
	return lx(x, y, id);
}


int main()
{
	int t, n, cnt, num;
	scanf("%d", &t);
	lx tmp1, tmp2;
	vectorans;
	while (t--)
	{
		num = 0;
		cnt = 0;
		ans.clear();
		scanf("%d", &n);
		for (int i = 1; i <= n; ++i)
		{
			scanf("%d%d", &tmp[i].x, &tmp[i].y);
			tmp[i].id = i;
		}
		sort(tmp + 1, tmp + n + 1);
		//for (int i = 1; i <= n; ++i)cout << tmp[i].x << " " << tmp[i].y << " " << tmp[i].id << endl;
		if (tmp[2].x >= tmp[1].x && tmp[2].x <= tmp[1].y)
		{
			tmp1 = tmp[1], tmp2 = tmp[2];
			num = 2;
		}
		else
		{
			tmp1 = tmp[2], num = 1;
		}
		for (int i = 3; i <= n; ++i)
		{
			if (num == 2)
			{
				if (tmp[i].x >= tmp2.x && tmp[i].x <= tmp2.y)
				{
					if (tmp[i].x >= tmp1.x && tmp[i].x <= tmp1.y)
					{
						cnt++;
						lx uu = getid(tmp[i], tmp1, tmp2);
						ans.push_back(uu.id);
						if (uu.id == tmp1.id)
						{
							tmp1 = tmp2; tmp2 = tmp[i];
						}
						else if (uu.id == tmp2.id)tmp2 = tmp[i];
					}
					else
					{
						tmp1 = tmp2;
						tmp2 = tmp[i];
					}
				}
				else
				{
					if (tmp[i].x >= tmp1.x && tmp[i].x <= tmp1.y)
					{
						tmp2 = tmp[i];
					}
					else
					{
						tmp1 = tmp[i]; num = 1;
					}
				}
			}
			else
			{
				if (tmp[i].x >= tmp1.x && tmp[i].x <= tmp1.y)
				{
					tmp2 = tmp[i]; num = 2;
				}
				else
				{
					tmp1 = tmp[i];
				}
			}
		}
		sort(ans.begin(), ans.end());
		printf("%d\n", cnt);
		if (cnt == 0)printf("\n");
		for (int i = 0; i < ans.size(); ++i)
		{
			if (i == ans.size() - 1)printf("%d\n", ans[i]);
			else printf("%d ", ans[i]);
		}
	}
}


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