[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。
  1 /*
  2 ━━━━━┒ギリギリ♂ eye！
  3 ┓┏┓┏┓┃キリキリ♂ mind！
  4 ┛┗┛┗┛┃＼○／
  5 ┓┏┓┏┓┃ /
  6 ┛┗┛┗┛┃ノ)
  7 ┓┏┓┏┓┃
  8 ┛┗┛┗┛┃
  9 ┓┏┓┏┓┃
 10 ┛┗┛┗┛┃
 11 ┓┏┓┏┓┃
 12 ┛┗┛┗┛┃
 13 ┓┏┓┏┓┃
 14 ┃┃┃┃┃┃
 15 ┻┻┻┻┻┻
 16 */
 17 #include <algorithm>
 18 #include <iostream>
 19 #include <iomanip>
 20 #include <cstring>
 21 #include <climits>
 22 #include <complex>
 23 #include <fstream>
 24 #include <cassert>
 25 #include <cstdio>
 26 #include <bitset>
 27 #include <vector>
 28 #include <deque>
 29 #include <queue>
 30 #include <stack>
 31 #include <ctime>
 32 #include <set>
 33 #include <map>
 34 #include <cmath>
 35 using namespace std;
 36 #define fr first
 37 #define sc second
 38 #define cl clear
 39 #define BUG puts("here!!!")
 40 #define W(a) while(a--)
 41 #define pb(a) push_back(a)
 42 #define Rint(a) scanf("%d", &a)
 43 #define Rll(a) scanf("%lld", &a)
 44 #define Rs(a) scanf("%s", a)
 45 #define Cin(a) cin >> a
 46 #define FRead() freopen("in", "r", stdin)
 47 #define FWrite() freopen("out", "w", stdout)
 48 #define Rep(i, len) for(int i = 0; i < (len); i++)
 49 #define For(i, a, len) for(int i = (a); i < (len); i++)
 50 #define Cls(a) memset((a), 0, sizeof(a))
 51 #define Clr(a, x) memset((a), (x), sizeof(a))
 52 #define Full(a) memset((a), 0x7f7f, sizeof(a))
 53 #define lrt rt << 1
 54 #define rrt rt << 1 | 1
 55 #define pi 3.14159265359
 56 #define RT return
 57 #define lowbit(x) x & (-x)
 58 #define onenum(x) __builtin_popcount(x)
 59 typedef long long LL;
 60 typedef long double LD;
 61 typedef unsigned long long ULL;
 62 typedef pair<int, int> pii;
 63 typedef pair<string, int> psi;
 64 typedef pair<LL, LL> pll;
 65 typedef map<string, int> msi;
 66 typedef vector<int> vi;
 67 typedef vector<LL> vl;
 68 typedef vector<vl> vvl;
 69 typedef vector<bool> vb;
 70 
 71 const int maxn = 505;
 72 int n, m;
 73 int pre[maxn];
 74 int in[maxn];
 75 
 76 int find(int x) {
 77     return x == pre[x] ? x : pre[x] = find(pre[x]);
 78 }
 79 
 80 void unite(int x, int y) {
 81     x = find(x); y = find(y);
 82     if(x != y) pre[x] = y;
 83 }
 84 
 85 int main() {
 86     // FRead();
 87     int T;
 88     int u, v;
 89     Rint(T);
 90     W(T) {
 91         Rint(n); Rint(m);
 92         bool exflag = 0; Cls(in);
 93         Rep(i, n+10) pre[i] = i;
 94         Rep(i, m) {
 95             Rint(u); Rint(v);
 96             in[v]++;
 97             unite(u, v);
 98         }
 99         int rt = find(pre[1]);
100         if(in[1] > 1) exflag = 1;
101         For(i, 2, n+1) {
102             if(find(pre[i]) != rt || in[i] > 1) {
103                 exflag = 1;
104                 break;
105             }
106         }
107         if(exflag) printf("NO
");
108         else printf("YES
");
109     }
110     RT 0;
111 }