[HIHO1322]树结构判定（并查集）

题目链接：http://hihocoder.com/problemset/problem/1322

给一个图，判断这个图是不是一棵树。

判定的方法：首先是连通图，其次所有点的入度都小于等于1。

 /*

 ━━━━━┒ギリギリ♂ eye！

 ┓┏┓┏┓┃キリキリ♂ mind！

 ┛┗┛┗┛┃＼○／

 ┓┏┓┏┓┃ /

 ┛┗┛┗┛┃ノ)

 ┓┏┓┏┓┃

 ┛┗┛┗┛┃

 ┓┏┓┏┓┃

 ┛┗┛┗┛┃

 ┓┏┓┏┓┃

 ┛┗┛┗┛┃

 ┓┏┓┏┓┃

 ┃┃┃┃┃┃

 ┻┻┻┻┻┻

 */

 #include <algorithm>

 #include <iostream>

 #include <iomanip>

 #include <cstring>

 #include <climits>

 #include <complex>

 #include <fstream>

 #include <cassert>

 #include <cstdio>

 #include <bitset>

 #include <vector>

 #include <deque>

 #include <queue>

 #include <stack>

 #include <ctime>

 #include <set>

 #include <map>

 #include <cmath>

 using namespace std;

 #define fr first

 #define sc second

 #define cl clear

 #define BUG puts("here!!!")

 #define W(a) while(a--)

 #define pb(a) push_back(a)

 #define Rint(a) scanf("%d", &a)

 #define Rll(a) scanf("%lld", &a)

 #define Rs(a) scanf("%s", a)

 #define Cin(a) cin >> a

 #define FRead() freopen("in", "r", stdin)

 #define FWrite() freopen("out", "w", stdout)

 #define Rep(i, len) for(int i = 0; i < (len); i++)

 #define For(i, a, len) for(int i = (a); i < (len); i++)

 #define Cls(a) memset((a), 0, sizeof(a))

 #define Clr(a, x) memset((a), (x), sizeof(a))

 #define Full(a) memset((a), 0x7f7f, sizeof(a))

 #define lrt rt << 1

 #define rrt rt << 1 | 1

 #define pi 3.14159265359

 #define RT return

 #define lowbit(x) x & (-x)

 #define onenum(x) __builtin_popcount(x)

 typedef long long LL;

 typedef long double LD;

 typedef unsigned long long ULL;

 typedef pair<int, int> pii;

 typedef pair<string, int> psi;

 typedef pair<LL, LL> pll;

 typedef map<string, int> msi;

 typedef vector<int> vi;

 typedef vector<LL> vl;

 typedef vector<vl> vvl;

 typedef vector<bool> vb;

 const int maxn = ;

 int n, m;

 int pre[maxn];

 int in[maxn];

 int find(int x) {

     return x == pre[x] ? x : pre[x] = find(pre[x]);

 }

 void unite(int x, int y) {

     x = find(x); y = find(y);

     if(x != y) pre[x] = y;

 }

 int main() {

     // FRead();

     int T;

     int u, v;

     Rint(T);

     W(T) {

         Rint(n); Rint(m);

         bool exflag = ; Cls(in);

         Rep(i, n+) pre[i] = i;

         Rep(i, m) {

             Rint(u); Rint(v);

             in[v]++;

             unite(u, v);

         }

         int rt = find(pre[]);

         if(in[] > ) exflag = ;

         For(i, , n+) {

             if(find(pre[i]) != rt || in[i] > ) {

                 exflag = ;

                 break;

             }

         }

         if(exflag) printf("NO\n");

         else printf("YES\n");

     }

     RT ;

 }