HDU1272&&POJ1308 小希的迷宫&&Is It A Tree?（并查集）

题意：

中文题不解释

要点：

两题一毛一样，判断是否合并后的结构是一棵树，即结构中应该没有环路，所以合并时如果发现两个根结点是相同的，就说明存在环路了。并且还要满足顶点数=边数+1。这题比较恶心的地方在于，如果一开始输入0，0要输出Yes，输入部分比较难写，而且HDU改题的时候还没提这事，POJ上倒是有说明。

HDU1272：

16711170

2016-03-29 13:50:08

Accepted

1272

62MS

3368K

1009 B

C++

seasonal

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#define maxn 100050
int p[maxn];
bool valid,mark[maxn];
int edge;

void init()
{
	valid = true;
	edge = 0;
	memset(mark, false, sizeof(mark));
	for (int i = 1; i < maxn; i++)
		p[i] = i;
}
int find(int x)
{
	if (p[x] == x) return x;
	return p[x] = find(p[x]);
}
void merge(int x, int y)
{
	int xx = find(x);
	int yy = find(y);
	if (xx == yy)
	{ 
		valid = false;
		return;
	}
	mark[x] = true;
	mark[y] = true;
	edge++;
	p[xx] = yy;
}

int main()
{
	int x, y;
	while (~scanf("%d%d", &x, &y))
	{
		if (x == -1 && y == -1)
			break;
		if (x == 0 && y == 0)	//一开始就是0，0必须输出Yes
		{
			printf("Yes\n");
			continue;
		}
		init();
		merge(x, y);
		while (~scanf("%d%d", &x, &y))
		{
			if (x == 0 && y == 0)
				break;
			merge(x, y);
		}
		int node = 0;
		for (int i = 1; i < maxn; i++)
			if (mark[i])
				node++;
		if (!valid || (node != edge + 1))
			printf("No\n");
		else
			printf("Yes\n");
	}
	return 0;
}

POJ1308：

15330358

Seasonal

1308

Accepted

828K

0MS

G++

1040B

2016-03-29 21:59:45

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#define maxn 100050
int p[maxn];
bool valid,mark[maxn];
int edge;

void init()
{
	valid = true;
	edge = 0;
	memset(mark, false, sizeof(mark));
	for (int i = 1; i < maxn; i++)
		p[i] = i;
}
int find(int x)
{
	if (p[x] == x) return x;
	return p[x] = find(p[x]);
}
void merge(int x, int y)
{
	int xx = find(x);
	int yy = find(y);
	if (xx == yy)
	{ 
		valid = false;
		return;
	}
	mark[x] = true;
	mark[y] = true;
	edge++;
	p[xx] = yy;
}

int main()
{
	int x, y,kase=1;
	while (~scanf("%d%d", &x, &y))
	{
		if (x == -1 && y == -1)
			break;
		if (x == 0 && y == 0)	//一开始就是0，0必须输出Yes
		{
			printf("Case %d is a tree.\n", kase++);
			continue;
		}
		init();
		merge(x, y);
		while (~scanf("%d%d", &x, &y))
		{
			if (x == 0 && y == 0)
				break;
			merge(x, y);
		}
		int node = 0;
		for (int i = 1; i < maxn; i++)
			if (mark[i])
				node++;
		if (!valid || (node != edge + 1))
			printf("Case %d is not a tree.\n",kase++);
		else
			printf("Case %d is a tree.\n",kase++);
	}
	return 0;
}