HDU 1272

• 无向图用并查集判断是否成环.
• 坑点：仅仅考虑了根节点相同(有回路)的情况,而忽略了有多个连通分量,也是不满足 任意两个房间有且仅有一条路径相同

#include <iostream>
#include <string.h>
using namespace std;

const int maxn = 1e5+10;

int pre[maxn];
int vis[maxn];
bool flag;
int maxnum;
void init() {
	for (int i=0; i<maxn; i++) pre[i] = i;
	memset(vis,0,sizeof vis);
	flag = true;
	maxnum = 0;
}
int find(int x) {
	if (x == pre[x]) return  x;
	return pre[x] = find(pre[x]);
}

void merge(int a,int b) {
	int ra = find(a);
	int rb = find(b);
	if (ra == rb) flag = false;
	else pre[ra] = rb;
}
int u,v;
int main() {
//	freopen("a.txt","r",stdin);
	init();
	while ( scanf("%d%d",&u,&v) ) {
		if ( u == -1 && v == -1 ) return 0;

		if ( u == 0 && v == 0 ) {
			int cnt = 0;
			for (int i=1; i<=maxnum; i++) 
				if (vis[i] && pre[i] == i) cnt++;
			
			if (flag && cnt<=1) printf("Yes\n");
			else printf("No\n");
			init();
			continue;
		}
		maxnum = max(maxnum , max(u,v));
		vis[u] = 1;
		vis[v] = 1;
		if (u != v) merge(u,v);
	}
	return 0;
}