UVA 796

题意：给你一个无向图，你需要找出里面的桥，并把所有桥按字典序输出

这一道题就是用无向图求桥的模板就可以了。

我一直错就是因为我在输入路径的时候少考虑一点

错误代码+原因：

  1 #include<stdio.h>
  2 #include<string.h>
  3 #include<iostream>
  4 #include<algorithm>
  5 #include<queue>
  6 using namespace std;
  7 const int maxn=1005;
  8 struct shudui
  9 {
 10     int x,y;
 11     bool operator < (const shudui t)const
 12     {
 13         if(x==t.x)
 14             return y>t.y;
 15         else return x>t.x;
 16     }
 17 }str1;
 18 priority_queue<shudui>r;
 19 int w[maxn][maxn];
 20 int cnt,head[maxn],n,dfn[maxn],low[maxn],num,qua,root,iscut[maxn];
 21 struct edge
 22 {
 23     int u,v,next;
 24 }e[maxn*maxn];
 25 void add_edge(int x,int y)
 26 {
 27     e[cnt].u=x;
 28     e[cnt].v=y;
 29     e[cnt].next=head[x];
 30     head[x]=cnt++;
 31 }
 32 void tarjan(int x,int fx)
 33 {
 34     dfn[x]=low[x]=++num;
 35     for(int i=head[x];i;i=e[i].next)
 36     {
 37         int to=e[i].v;
 38         if(!dfn[to])
 39         {
 40             tarjan(to,i);
 41             low[x]=min(low[x],low[to]);
 42             if(low[to]>dfn[x])
 43             {  //因为我把双向边放在了一起，即1->2放在了cnt为2的地方，2->1放在了cnt为3的地方。
 44                //那么就可以通过2^1=3找到另一个反方向的边，或者3^1=2.那么这个双向边就被标记成了桥
 45                 iscut[i]=iscut[i^1]=1;
 46             }
 47         }
 48         else if(i!=(fx^1))
 49             low[x]=min(dfn[to],low[x]);
 50     }
 51 }
 52 int main()
 53 {
 54     while(~scanf("%d",&n))
 55     {
 56         if(n==0)
 57         {
 58             printf("0 critical links
");
 59             continue;
 60         }
 61         cnt=2;
 62         num=qua=0;
 63         memset(w,0,sizeof(w));
 64         memset(iscut,0,sizeof(iscut));
 65         memset(head,0,sizeof(head));
 66         memset(dfn,0,sizeof(dfn));
 67         memset(low,0,sizeof(low));
 68         for(int i=1;i<=n;++i)
 69         {
 70             int x,y,sum;
 71             char ch;
 72             scanf("%d %c%d%c",&x,&ch,&sum,&ch);
 73             x+=1;
 74             while(sum--)
 75             {
 76                 scanf("%d",&y);
 77                 y+=1;
 78                 if(!w[x][y])  //我原本是想要添加就添加一对互反方向的边(即1->2和2->1，因为用这种方式找桥的时
 79                 //候，你必须保证互反方向的边存在一起，这样可以通过i^1来找到边的另一个方向)。但是我有一点没有考虑到
 80                 //就是我这样判断的话如果题目给出两条1到2的无向边的时候我的代码就错了。因为我的代码默认
 81                 //只会给两点之间添加至多一次无向边
 82                 {
 83                     add_edge(x,y);
 84                     add_edge(y,x);
 85                     w[x][y]=w[y][x]=1;
 86                 }
 87             }
 88         }
 89 
 90         for(int i=1;i<=n;++i)
 91         {
 92             if(!dfn[i])
 93             {
 94                 tarjan(i,0);
 95             }
 96         }
 97         int ans=0;
 98         for(int i=2;i<cnt;i+=2)
 99         {
100             if(iscut[i])
101             {
102                 str1.x=min(e[i^1].v-1,e[i].v-1);
103                 str1.y=max(e[i^1].v-1,e[i].v-1);
104                 r.push(str1);
105                 ans++;
106             }
107 
108         }
109         printf("%d critical links
",ans);
110         while(!r.empty())
111         {
112             str1=r.top();
113             r.pop();
114             printf("%d - %d
",str1.x,str1.y);
115         }
116         printf("
");
117     }
118     return 0;
119 }

View Code

正解代码1：

  1 #include<stdio.h>
  2 #include<string.h>
  3 #include<iostream>
  4 #include<algorithm>
  5 #include<queue>
  6 #include<map>
  7 #include<vector>
  8 using namespace std;
  9 const int maxn = 1e5+10;
 10 const int M=1e5+10;
 11 struct node {
 12     int v, next;
 13 }e[M];
 14 struct shudui
 15 {
 16     int x,y;
 17     bool operator < (const shudui t)const
 18     {
 19         if(x==t.x)
 20             return y>t.y;
 21         else return x>t.x;
 22     }
 23 }str1;
 24 priority_queue<shudui>r;
 25 map<int,int>w;
 26 int head[maxn], cnt;
 27 bool iscut[maxn];
 28 int dfn[maxn], low[maxn];
 29 int num,n;
 30 inline void add_edge(int xx, int yy) {
 31     e[++cnt].next = head[xx];
 32     e[cnt].v = yy;
 33     head[xx] = cnt;
 34 }
 35 
 36 inline void tarjan(int x, int in_edge) {
 37     dfn[x] = low[x] = ++num;
 38     for(int i = head[x]; i; i = e[i].next) {
 39         int to = e[i].v;
 40         if(!dfn[to]) {
 41             tarjan(to, i);
 42             low[x] = min(low[x], low[to]);
 43             if(low[to] > dfn[x])
 44                 iscut[i] = iscut[i ^ 1] = true;
 45         }
 46         else if(i != (in_edge ^ 1))
 47             low[x] = min(low[x], dfn[to]);
 48     }
 49 }
 50 int main()
 51 {
 52     while(~scanf("%d",&n))
 53     {
 54         cnt=1;
 55         num=0;
 56         memset(iscut,0,sizeof(iscut));
 57         memset(head,0,sizeof(head));
 58         memset(dfn,0,sizeof(dfn));
 59         memset(low,0,sizeof(low));
 60         if(n==0)
 61         {
 62             printf("0 critical links

");
 63             continue;
 64         }
 65         int u,k,v;
 66         for(int i = 1; i <= n; i++)
 67         {
 68             scanf("%d (%d)",&u,&k);
 69             u++;
 70             while(k--)
 71             {
 72                 scanf("%d",&v);
 73                 v++;
 74                 if(v <= u)continue;
 75                 add_edge(u,v);
 76                 add_edge(v,u);
 77             }
 78         }
 79         for(int i=1;i<=n;++i)
 80         {
 81             if(!dfn[i])
 82             {
 83                 tarjan(i,0);
 84             }
 85         }
 86         vector<pair<int,int> >ans;
 87         int anss=0;
 88         for(int i=2;i<cnt;i+=2)
 89         {
 90             if(iscut[i])
 91             {
 92                 int xx=min(e[i^1].v-1,e[i].v-1);
 93                 int yy=max(e[i^1].v-1,e[i].v-1);
 94                 ans.push_back(make_pair(xx,yy));
 95                 anss++;
 96             }
 97 
 98         }
 99         printf("%d critical links
",anss);
100         sort(ans.begin(),ans.end());  //默认从小到大排序
101         for(int i = 0; i < ans.size(); i++)
102             printf("%d - %d
",ans[i].first,ans[i].second); 
103         printf("
");
104     }
105     return 0;
106 }

View Code

正解代码2(kuangbin)：

  1 #include <cstdio>
  2 #include <iostream>
  3 #include <cstring>
  4 #include <algorithm>
  5 #include <map>
  6 #include <vector>
  7 using namespace std;
  8 const int N = 1e4+10;
  9 const int M = 1e5+10;
 10 struct Edge{
 11     int to,next;
 12     bool cut;//是否为桥的标记
 13 }edge[M];
 14 int head[N],tot,n;
 15 int low[N],dfn[N],Stack[N];
 16 int Index,top;
 17 bool Instack[N],cut[N];
 18 int add_block[N];//删除一个点后增加的连通块
 19 int bridge;
 20 void init(){
 21     memset(head,-1,sizeof(head));
 22     memset(dfn,0,sizeof(dfn));
 23     memset(Instack,false,sizeof(Instack));
 24     memset(add_block,0,sizeof(add_block));
 25     memset(cut,false,sizeof(cut));
 26     Index=top=bridge=tot = 0;
 27 }
 28 void addedge(int u,int v){
 29     edge[tot].to = v;edge[tot].next = head[u];edge[tot].cut = false;
 30     head[u] = tot++;
 31 }
 32 void Tarjan(int u,int pre){
 33     low[u] = dfn[u] = ++Index;
 34     Stack[top++] = u;
 35     Instack[u] = true;     
 36     int son = 0;
 37     for(int i = head[u];i != -1;i = edge[i].next){
 38         int v = edge[i].to;
 39         if(v == pre)continue;
 40         if( !dfn[v] ){
 41             son++;
 42             Tarjan(v,u);
 43             if(low[u] > low[v])low[u] = low[v];         
 44             if(low[v] > dfn[u]){    //一条无向边(u,v)是桥，当且仅当(u,v)为树枝边，且满足DFS(u)<low(v)
 45                 bridge++;
 46                 edge[i].cut = true;     //正反两边都标记为桥
 47                 edge[i^1].cut = true;
 48             }
 49             if(u != pre && low[v] >= dfn[u]){         
 50                 cut[u] = true;   //该点为割点 
 51                 add_block[u]++;
 52             }
 53         }
 54         else if( low[u] > dfn[v])low[u] = dfn[v];
 55     }
 56     if(u == pre && son > 1)cut[u] = true;  //若u为根，且分支数>1，则u割点
 57     if(u == pre)add_block[u] = son - 1;    
 58     Instack[u] = false;
 59     top--;
 60 }
 61 void solve(){
 62     for(int i = 1;i <= n;i++)
 63         if( !dfn[i] )
 64             Tarjan(i,i);
 65     printf("%d critical links
",bridge);
 66     
 67     vector<pair<int,int> >ans;          /*--   将桥按顺序输出  --*/
 68     for(int u = 1;u <= n;u++)
 69         for(int i = head[u];i != -1;i = edge[i].next)
 70             if(edge[i].cut && edge[i].to > u){
 71                 ans.push_back(make_pair(u,edge[i].to));
 72             }
 73     sort(ans.begin(),ans.end());
 74     for(int i = 0;i < ans.size();i++)
 75         printf("%d - %d
",ans[i].first-1,ans[i].second-1);
 76     printf("
");
 77 }
 78 //处理重边
 79 /*map<int,int>mapit;
 80 inline bool isHash(int u,int v)
 81 {
 82     if(mapit[u*N+v])return true;
 83     if(mapit[v*N+u])return true;
 84     mapit[u*N+v] = mapit[v*N+u] = 1;
 85     return false;
 86 }*/
 87 int main(){
 88     while(scanf("%d",&n) == 1){
 89         init();
 90         int u,k,v;
 91         //mapit.clear();
 92         for(int i = 1;i <= n;i++){
 93             scanf("%d (%d)",&u,&k);
 94             u++;
 95             //这样加边，要保证正边和反边是相邻的，建无向图
 96             while(k--){
 97                 scanf("%d",&v);
 98                 v++;
 99                 if(v <= u)continue;
100                 //if(isHash(u,v))continue;
101                 addedge(u,v);
102                 addedge(v,u);
103             }
104         }
105         solve();
106     }
107     return 0;
108 }

View Code

只求桥板子1：

 1 #include<bits/stdc++.h>
 2 using namespace std;
 3 const int maxn = 100086;
 4 struct node {
 5     int y, net;
 6 }e[maxn << 1];
 7 int lin[maxn], len = 1;
 8 bool bridge[maxn << 1];
 9 int dfn[maxn], low[maxn];
10 int n, m, num;
11 
12 inline int read() {
13     int x = 0, y = 1;
14     char ch = getchar();
15     while(!isdigit(ch)) {
16         if(ch == '-') y = -1;
17         ch = getchar();
18     }
19     while(isdigit(ch)) {
20         x = (x << 1) + (x << 3) + ch - '0';
21         ch = getchar();
22     }
23     return x * y;
24 }
25 
26 inline void insert(int xx, int yy) {
27     e[++len].net = lin[xx];
28     e[len].y = yy;
29     lin[xx] = len;
30 }
31 
32 inline void tarjan(int x, int in_edge) {
33     dfn[x] = low[x] = ++num;
34     for(int i = lin[x]; i; i = e[i].net) {
35         int to = e[i].y;
36         if(!dfn[to]) {
37             tarjan(to, i);
38             low[x] = min(low[x], low[to]);
39             if(low[to] > dfn[x])
40                 bridge[i] = bridge[i ^ 1] = true;
41         }
42         else if(i != (in_edge ^ 1))
43             low[x] = min(low[x], dfn[to]);
44     }
45 } 
46 
47 int main() {
48     n = read(), m = read();
49     len = 1;
50     for(int i = 1; i <= m; ++i) {
51         int x, y, z;
52         x = read(), y = read();
53         insert(x, y);
54         insert(y, x);
55     }
56     for(int i = 1; i <= n; ++i)
57         if(!dfn[i]) tarjan(i, 0);
58     for(int i = 2; i < len; i += 2) 
59         if(bridge[i])
60             cout << e[i ^ 1].y << ' ' << e[i].y << '
';
61     return 0;
62 }

View Code

只求桥板子2：

  1 #include <cstdio>
  2 #include <iostream>
  3 #include <cstring>
  4 #include <algorithm>
  5 #include <map>
  6 #include <vector>
  7 using namespace std;
  8 const int N = 1e4+10;
  9 const int M = 1e5+10;
 10 struct Edge{
 11     int to,next;
 12     bool cut;//是否为桥的标记
 13 }edge[M];
 14 int head[N],tot,n;
 15 int low[N],dfn[N],Stack[N];
 16 int Index,top;
 17 bool Instack[N],cut[N];
 18 int add_block[N];//删除一个点后增加的连通块
 19 int bridge;
 20 void init(){
 21     memset(head,-1,sizeof(head));
 22     memset(dfn,0,sizeof(dfn));
 23     memset(Instack,false,sizeof(Instack));
 24     memset(add_block,0,sizeof(add_block));
 25     memset(cut,false,sizeof(cut));
 26     Index=top=bridge=tot = 0;
 27 }
 28 void addedge(int u,int v){
 29     edge[tot].to = v;edge[tot].next = head[u];edge[tot].cut = false;
 30     head[u] = tot++;
 31 }
 32 void Tarjan(int u,int pre){
 33     low[u] = dfn[u] = ++Index;
 34 //    Stack[top++] = u;
 35 //    Instack[u] = true;
 36     int son = 0;
 37     for(int i = head[u];i != -1;i = edge[i].next){
 38         int v = edge[i].to;
 39         if(v == pre)continue;
 40         if( !dfn[v] ){
 41             son++;
 42             Tarjan(v,u);
 43             if(low[u] > low[v])low[u] = low[v];
 44             if(low[v] > dfn[u]){    //一条无向边(u,v)是桥，当且仅当(u,v)为树枝边，且满足DFS(u)<low(v)
 45                 bridge++;
 46                 edge[i].cut = true;     //正反两边都标记为桥
 47                 edge[i^1].cut = true;
 48             }
 49 //            if(u != pre && low[v] >= dfn[u]){
 50 //                cut[u] = true;   //该点为割点
 51 //                add_block[u]++;
 52 //            }
 53         }
 54         else if( low[u] > dfn[v])low[u] = dfn[v];
 55     }
 56 //    if(u == pre && son > 1)cut[u] = true;  //若u为根，且分支数>1，则u割点
 57 //    if(u == pre)add_block[u] = son - 1;
 58 //    Instack[u] = false;
 59 //    top--;
 60 }
 61 void solve(){
 62     for(int i = 1;i <= n;i++)
 63         if( !dfn[i] )
 64             Tarjan(i,i);
 65     printf("%d critical links
",bridge);
 66 
 67     vector<pair<int,int> >ans;          /*--   将桥按顺序输出  --*/
 68     for(int u = 1;u <= n;u++)
 69         for(int i = head[u];i != -1;i = edge[i].next)
 70             if(edge[i].cut && edge[i].to > u){
 71                 ans.push_back(make_pair(u,edge[i].to));
 72             }
 73     sort(ans.begin(),ans.end());
 74     for(int i = 0;i < ans.size();i++)
 75         printf("%d - %d
",ans[i].first-1,ans[i].second-1);
 76     printf("
");
 77 }
 78 //处理重边
 79 /*map<int,int>mapit;
 80 inline bool isHash(int u,int v)
 81 {
 82     if(mapit[u*N+v])return true;
 83     if(mapit[v*N+u])return true;
 84     mapit[u*N+v] = mapit[v*N+u] = 1;
 85     return false;
 86 }*/
 87 int main(){
 88     while(scanf("%d",&n) == 1){
 89         init();
 90         int u,k,v;
 91         //mapit.clear();
 92         for(int i = 1;i <= n;i++){
 93             scanf("%d (%d)",&u,&k);
 94             u++;
 95             //这样加边，要保证正边和反边是相邻的，建无向图
 96             while(k--){
 97                 scanf("%d",&v);
 98                 v++;
 99                 if(v <= u)continue;
100                 //if(isHash(u,v))continue;
101                 addedge(u,v);
102                 addedge(v,u);
103             }
104         }
105         solve();
106     }
107     return 0;
108 }

View Code