poj 2723

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <algorithm>
 5 #include <stack>
 6 #include <vector>
 7 
 8 using namespace std;
 9 
10 const int MAX_N = (1 << 11) + 5;
11 int N,M;
12 int low[MAX_N],pre[MAX_N],cmp[MAX_N];
13 int dfs_clock,scc_cnt;
14 stack <int > S;
15 int no[MAX_N],x[3000],y[3000];
16 vector<int> G[MAX_N];
17 
18 
19 void dfs(int u) {
20         pre[u] = low[u] = ++dfs_clock;
21         S.push(u);
22         for(int i = 0; i < G[u].size(); ++i) {
23                 int v = G[u][i];
24                 if(!pre[v]) {
25                         dfs(v);
26                         low[u] = min(low[u],low[v]);
27                 } else if(!cmp[v]) {
28                         low[u] = min(low[u],pre[v]);
29                 }
30         }
31 
32         if(low[u] == pre[u]) {
33                 ++scc_cnt;
34                 for(;;) {
35                         int x = S.top(); S.pop();
36                         cmp[x] = scc_cnt;
37                         if(x == u) break;
38                 }
39         }
40 }
41 void scc() {
42         dfs_clock = scc_cnt = 0;
43         memset(cmp,0,sizeof(cmp));
44         memset(pre,0,sizeof(pre));
45 
46         for(int i = 0; i < 2 * N; ++i) {
47                 if(!pre[i]) dfs(i);
48         }
49 }
50 bool check(int m) {
51         for(int i = 0; i < 2 * N; ++i) {
52                 G[i].clear();
53         }
54         for(int i = 1; i <= m; ++i) {
55                 int a = x[i],b = y[i];
56                 G[no[a]].push_back(b);
57                 G[no[b]].push_back(a);
58         }
59         scc();
60         for(int i = 0; i < 2 * N; ++i) {
61                 if(cmp[i] == 0) continue;
62                 if(cmp[i] == cmp[ no[i] ]) {
63                         //printf("i = %d no = %d %d %d\n",i,no[i],cmp[i],cmp[no[i]]);
64                         return false;
65                 }
66         }
67 
68         return true;
69 }
70 void solve() {
71         int l = 0,r = M;
72         while(l < r) {
73                 int mid = (l + r + 1) / 2;
74                 if(check(mid)) l = mid;
75                 else r = mid - 1;
76         }
77         printf("%d\n",l);
78 }
79 int main()
80 {
81     //freopen("sw.in","r",stdin);
82     while(~scanf("%d%d",&N,&M)) {
83             if(N == 0 && M == 0) break;
84             for(int i = 1; i <= N; ++i) {
85                     int a,b;
86                     scanf("%d%d",&a,&b);
87                     no[a] = b;
88                     no[b] = a;
89             }
90             for(int i = 1; i <= M; ++i) {
91                     scanf("%d%d",&x[i],&y[i]);
92             }
93 
94             solve();
95 
96     }
97     return 0;
98 }