uva-796.critical links(连通图的桥)

本题大意:求出一个无向图的桥的个数并且按照顺序输出所有桥.

本题思路:注意判重就行了,就是一个桥的裸题.

　　判重思路目前知道的有两种,第一种是哈希判重,第二种和邻接矩阵的优化一样,就是只存图的上半角或者下半角.

参考代码:

 /*************************************************************************
     > File Name: uva-796.critical_links.cpp
     > Author: CruelKing
     > Mail: 2016586625@qq.com
     > Created Time: 2019年09月06日 星期五 15时58分54秒
     本题思路:注意边的判重.
  ************************************************************************/

 #include <cstdio>
 #include <cstring>
 #include <vector>
 #include <map>
 #include <algorithm>
 using namespace std;

 const int maxn =  + , maxm = maxn * maxn + ;
 int n;
 struct Edge {
     int to, next;
     bool cut;
 } edge[maxm];
 int head[maxn], tot;
 int low[maxn], dfn[maxn],stack[maxn];
 int Index, top, bridge;
 bool instack[maxn];
 bool cut[maxn];
 int add_block[maxn];

 void addedge(int u, int v) {
     edge[tot].to = v; edge[tot].next = head[u]; edge[tot].cut = false;
     head[u] = tot ++;
 }

 void init() {
     memset(head, -, sizeof head);
     tot = ;
 }

 map<int, int> mp;
 vector <pair<int, int> > ans;

 void tarjan(int u, int pre) {
     int v;
     low[u] = dfn[u] = ++ Index;
     instack[u] = true;
     int son = ;
     int pre_cnt = ;
     for(int i = head[u]; ~i; i = edge[i].next) {
         v = edge[i].to;
         if(v == pre && pre_cnt == ) {
             pre_cnt ++;
             continue;
         }
         if(!dfn[v]) {
             son ++;
             tarjan(v, u);
             if(low[u] > low[v]) low[u] = low[v];
             if(low[v] > dfn[u]) {
                 bridge ++;
                 edge[i].cut = true;
                 edge[i ^ ].cut = true;
             }
             if(u != pre && low[v] >= dfn[u]) {
                 cut[u] = true;
                 add_block[u] ++;
             }
         } else if(low[u] > dfn[v]) low[u] = dfn[v];
     }
     if(u == pre && son > ) cut[u] = true;
     if(u == pre) add_block[u] = son - ;
     instack[u] = false;
     top --;
 }

 void solve() {
     memset(dfn, , sizeof dfn);
     memset(instack, false, sizeof instack);
     memset(add_block, , sizeof add_block);
     memset(cut, false, sizeof cut);
     Index = top = bridge = ;
     ans.clear();
     for(int i = ; i < n; i ++)
         if(!dfn[i])
             tarjan(i, i);
     printf("%d critical links\n", bridge);
     for(int u = ; u < n; u ++) {
         for(int i = head[u]; ~i; i = edge[i].next) {
             if(edge[i].cut && edge[i].to > u) ans.push_back(make_pair(u, edge[i].to));
         }
     }
     sort(ans.begin(), ans.end());
     for(int i = ; i < ans.size(); i ++)
         printf("%d - %d\n", ans[i].first, ans[i].second);
     printf("\n");
 }

 inline bool ishash(int u, int v) {
     return !(mp[u * maxn + v]++ || mp[v * maxn + u]++);
 }

 int main() {
     int u, num, v;
     while(scanf("%d", &n) == ) {
         mp.clear();
         init();
         for(int i = ; i < n; i ++) {
             scanf("%d (%d)", &u, &num);
             for(int i = ; i < num; i ++) {
                 scanf("%d", &v);
                 if(ishash(u, v)) continue;
                 addedge(u, v);
                 addedge(v, u);
             }
         }
         solve();
     }
     return ;
 }