【HDOJ】4612 Warm up

双连通缩点+求树的直径，图论基础题目。

 /* 4612 */
 #pragma comment(linker, "/STACK:1024000000,1024000000")
 #include <iostream>
 #include <algorithm>
 #include <vector>
 #include <queue>
 #include <cstdio>
 #include <cstring>
 #include <cstdlib>
 using namespace std;

 #define MAXV 200005
 #define MAXE 1000005

 typedef struct {
     int v, next;
 } Edge_t;

 Edge_t E[MAXE*];
 vector<int> vc[MAXV];
 int low[MAXV], pre[MAXV];
 int bn[MAXV];
 int S[MAXV], top;
 int head[MAXV], L;
 bool visit[MAXV];
 int dfs_clock, block;
 int n, m;

 void addEdge(int u, int v) {
     E[L].v = v;
     E[L].next = head[u];
     head[u] = L++;
 }

 void init() {
     memset(head, -, sizeof(int)*(n+));
     memset(pre, , sizeof(int)*(n+));
     memset(bn, , sizeof(int)*(n+));
     dfs_clock = block = L = top = ;
 }

 void tarjan(int u, int fa) {
     int i, j, k, v;

     S[top++] = u;
     pre[u] = low[u] = ++dfs_clock;
     for (i=head[u]; i!=-; i=E[i].next) {
         if (i == (fa^))
             continue;
         v = E[i].v;
         if (!pre[v]) {
             tarjan(v, i);
             low[u] = min(low[u], low[v]);
         } else if (!bn[v]) {
             low[u] = min(low[u], pre[v]);
         }
     }
     if (low[u] == pre[u]) {
         ++block;
         do {
             bn[S[--top]] = block;
         } while (S[top]!=u);
     }
 }

 void solve() {
     queue<int> Q;
     int i, j, k;
     int u = , v;

     memset(pre, , sizeof(int)*(block+));
     pre[u] = ;
     Q.push(u);
     while (!Q.empty()) {
         u = Q.front();
         Q.pop();
         for (i=; i<vc[u].size(); ++i) {
             v = vc[u][i];
             if (!pre[v]) {
                 pre[v] = ;
                 Q.push(v);
             }
         }
     }

     memset(pre, , sizeof(int)*(block+));
     pre[u] = ;
     Q.push(u);
     while (!Q.empty()) {
         u = Q.front();
         Q.pop();
         k = pre[u];
         for (i=; i<vc[u].size(); ++i) {
             v = vc[u][i];
             if (!pre[v]) {
                 pre[v] = k+;
                 Q.push(v);
             }
         }
     }

     printf("%d\n", block-pre[u]);
 }

 int main() {
     int i, j, k;

     #ifndef ONLINE_JUDGE
         freopen("data.in", "r", stdin);
         freopen("data.out", "w", stdout);
     #endif

     while (scanf("%d %d",&n,&m)!=EOF && (n||m)) {
         init();
         for (i=; i<m; ++i) {
             scanf("%d %d", &j, &k);
             addEdge(j, k);
             addEdge(k, j);
         }
         for (i=; i<=n; ++i)
             if (!pre[i])
                 tarjan(i, -);
         for (i=; i<=block; ++i)
             vc[i].clear();
         for (i=; i<=n; ++i) {
             for (j=head[i]; j!=-; j=E[j].next) {
                 k = E[j].v;
                 if (bn[i] != bn[k]) {
                     // bridge
                     vc[bn[i]].push_back(bn[k]);
                     vc[bn[k]].push_back(bn[i]);
                 }
             }
         }
         solve();
     }

     return ;
 }