HDU 3062：Party（2-SAT入门）

http://acm.hdu.edu.cn/showproblem.php?pid=3062

题意：中文。

思路：裸的2-SAT。判断二元组的两个人是否在同一个强连通分量。

学习地址：http://www.cnblogs.com/ambition/archive/2011/07/30/2-sat.html

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 #define N 2010
 4 struct Edge {
 5     int u, v, nxt;
 6 } edge[N*N*2];
 7 int head[N], tot, belong[N], num, dfn[N], low[N], vis[N], tid;
 8 stack<int> sta;
 9
10 void Add(int u, int v) {
11     edge[tot] = (Edge) { u, v, head[u] }; head[u] = tot++;
12 }
13
14 void init() {
15     memset(head, -1, sizeof(head));
16     memset(vis, 0, sizeof(vis));
17     memset(dfn, 0, sizeof(dfn));
18     memset(low, 0, sizeof(low));
19     tot = num = tid = 0;
20     while(!sta.empty()) sta.pop();
21 }
22
23 void tarjan(int u) {
24     dfn[u] = low[u] = ++tid;
25     vis[u] = 1; sta.push(u);
26     for(int i = head[u]; ~i; i = edge[i].nxt) {
27         int v = edge[i].v;
28         if(!dfn[v]) {
29             tarjan(v);
30             if(low[u] > low[v]) low[u] = low[v];
31         } else if (vis[v]) {
32             if(low[u] > dfn[v]) low[u] = dfn[v];
33         }
34     }
35     if(low[u] == dfn[u]) {
36         num++;
37         while(true) {
38             int x = sta.top(); sta.pop();
39             belong[x] = num;
40             vis[x] = 0;
41             if(x == u) break;
42         }
43     }
44 }
45
46 int main() {
47     int n, m;
48     while(~scanf("%d", &n)) {
49         scanf("%d", &m); init();
50         for(int i = 0; i < m; i++) {
51             int a, b, c, d;
52             scanf("%d%d%d%d", &a, &b, &c, &d);
53             Add(a * 2 + c, b * 2 + (d + 1) % 2);
54             Add(b * 2 + d, a * 2 + (c + 1) % 2);
55         }
56         for(int i = 0; i < 2 * n; i++) if(!dfn[i]) tarjan(i);
57         bool flag = 1;
58         for(int i = 0; i < n; i++)
59             if(belong[i*2] == belong[i*2+1]) flag = 0;
60         if(flag) puts("YES");
61         else puts("NO");
62     }
63     return 0;
64 }