Codeforces 771A Bear and Friendship Condition[并查集]

题意:给了你一张图,问是否 对任意的A,B,C,有A-B 和A-C的话,同时有B-C 

分析:其实就是每个块都是一个完全图,n个顶点的完全图有n*(n-1)/2条边。用并查集统计每个块有几个点,几条边。

需要注意的是n*(n-1)/2这里是会爆int的,需要用longlong(心痛。。

以下是代码:

#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 = 1005;
const int MAXM = 5005;
const ll LINF = 0x3f3f3f3f3f3f3f3f;
const int INF = 0x3f3f3f3f;
const int MOD = 1000000007;
const double FINF = 1e18;
int pre[150005];
int find(int x) { return x == pre[x] ? x : pre[x] = find(pre[x]); }
void mix(int a, int b)
{
	int fa = find(a), fb = find(b);
	if (fa != fb)
		pre[fa] = fb;
}
int x[150005], y[150005];
int cnt[150005];
int pp[150005];
int main()
{
	ll n, m;
	memset(cnt, 0, sizeof(cnt));
	memset(pp, 0, sizeof(pp));
	cin >> n >> m;
	for (int i = 1; i <= n; ++i)pre[i] = i;
	for (int i = 0; i < m; ++i)
	{
		scanf("%d%d", &x[i], &y[i]);
		mix(x[i], y[i]);
	}
	for (int i = 0; i < m; ++i)
	{
		cnt[find(x[i])]++;
	}
	for (int i = 1; i <= n; ++i)
	{
		pp[find(i)]++;
	}
	for (int i = 1; i <= n; ++i)
	{
		if (find(i) == i)
		{
			if (pp[i] *1LL* (pp[i] - 1) / 2 != cnt[i])
			{

				printf("NO\n");
				return 0;
			}
		}
	}
	printf("YES\n");
}


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