HDU 3062 Party

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <cmath>
 5 #include <algorithm>
 6 #include <climits>
 7 #include <vector>
 8 #include <queue>
 9 #include <cstdlib>
10 #include <string>
11 #include <set>
12 #include <stack>
13 #define LL long long
14 #define pii pair<int,int>
15 #define INF 0x3f3f3f3f
16 using namespace std;
17 const int maxn = 2010;
18 struct arc{
19     int u,v,next;
20     arc(int x = 0,int y = 0,int z = 0){
21         u = x;
22         v = y;
23         next = z;
24     }
25 };
26 arc e[2000100];
27 int dfn[maxn],low[maxn],belong[maxn];
28 bool instack[maxn];
29 int head[maxn],tot,scc,index,n,m;
30 stack<int>stk;
31 void add(int u,int v){
32     e[tot] = arc(u,v,head[u]);
33     head[u] = tot++;
34 }
35 void tarjan(int u){
36     dfn[u] = low[u] = ++index;
37     instack[u] = true;
38     stk.push(u);
39     for(int i = head[u]; ~i; i = e[i].next){
40         if(!dfn[e[i].v]){
41             tarjan(e[i].v);
42             if(low[e[i].v] < low[u])
43                 low[u] = low[e[i].v];
44         }else if(instack[e[i].v] && low[u] > dfn[e[i].v])
45             low[u] = dfn[e[i].v];
46     }
47     if(dfn[u] == low[u]){
48         scc++;
49         int now;
50         do{
51             now = stk.top();
52             stk.pop();
53             belong[now] = scc;
54             instack[now] = false;
55         }while(now != u);
56     }
57 
58 }
59 bool solve(){
60     int k = n<<1;
61     while(!stk.empty()) stk.pop();
62     for(int i = 0; i < k; i++)
63         if(!dfn[i]) tarjan(i);
64     for(int i = 0; i < n; i++)
65         if(belong[i<<1] == belong[i<<1|1]) return false;
66     return true;
67 }
68 int main() {
69     int u,v,x,y;
70     while(~scanf("%d %d",&n,&m)){
71         for(int i = 0; i < maxn; i++){
72             dfn[i] = belong[i] = 0;
73             head[i] = -1;
74             instack[i] = false;
75         }
76         tot = scc = index = 0;
77         for(int i = 0; i < m; i++){
78             scanf("%d %d %d %d",&x,&y,&u,&v);
79             x = (x<<1)+u;
80             y = (y<<1)+v;
81             add(x,y^1);
82             add(y,x^1);
83         }
84         solve()?puts("YES"):puts("NO");
85     }
86     return 0;
87 }