poj 1144 求割点 太奇怪了。。。。

求割点数很简单

难在一些细节

到现在还不清楚为什么一开始就设置dfn为1为什么会错

错误的代码

View Code

#include<stdio.h>
#include<string.h>
int n,son,ans;
int map[1000][1000];
int dfn[1000],low[1000];
int vis[1000];
int tdfn;
int flag[1000];
int min(int a,int b){
    return a<b?a:b;
}
void dfs(int u){
    int v;
    for(v=1;v<=n;v++){
        if(map[u][v]){
            if(!vis[v]){
                vis[v]=1;
                tdfn++;
                low[v]=dfn[v]=tdfn;
                dfs(v);//to get low[v];
                low[u]=min(low[u],low[v]);
                if(low[v]>=dfn[u])    {
                    if(u==1) son++;
                    else flag[u]=1;
                }
            }
            else low[u]=min(dfn[v],low[u]);
        }
    }
} 
void init(){
    tdfn=1;ans=0;vis[1]=1;son=0;
    memset(map,0,sizeof(map));
    memset(vis,0,sizeof(vis));
    memset(flag,0,sizeof(flag));
    dfn[1]=low[1]=1;
}
int a[1000];
int main(){
    int i=0,j;
    int u,v;
    char ch;
    while(scanf("%d",&n),n){
        init();
        while(scanf("%d",&u),u)
            while(getchar()!='\n')
            {
                scanf("%d",&v);
                map[u][v]=map[v][u]=1;
            }
        dfs(1);ans=0;
        if(son>=2) ans++;
        for(i=1;i<=n;i++) ans+=flag[i];
            printf("%d\n",ans);
    }
    return 0;
}

真确的代码

View Code

#include<stdio.h>
#include<string.h>
int n,son,ans;
int map[110][110];
int dfn[110],low[110];
int vis[110];
int tdfn;
int flag[110];
int min(int a,int b){
    return a<b?a:b;
}
void dfs(int u){
    int v;dfn[u]=low[u]=++tdfn;
    for(v=1;v<=n;v++){
        if(map[u][v]){
            if(!dfn[v]){
                dfs(v);//to get low[v];
                low[u]=min(low[u],low[v]);
                if(low[v]>=dfn[u])    {
                    if(u==1) son++;
                    else flag[u]=1;
                }
            }
            else low[u]=min(dfn[v],low[u]);
        }
    }
} 
void init(){
    tdfn=0;son=0;ans=0;
    memset(map,0,sizeof(map));
    memset(flag,0,sizeof(flag));
    memset(dfn,0,sizeof(dfn));
}
int a[110];
int main(){
    int i=0,j;
    int u,v;
    while(scanf("%d",&n),n){
        init();
        while(scanf("%d",&u),u)
            while(getchar()!='\n')
            {
                scanf("%d",&v);
                map[u][v]=map[v][u]=1;
            }
        dfs(1);
        if(son>=2) ans++;
        for(i=2;i<=n;i++) ans+=flag[i];
            printf("%d\n",ans);
    }
    return 0;
}

邻接表写法

#include<stdio.h>#include<string.h>#include<iostream>#include<vector>using namespace std;vector<int>list[1000];int low[1000],dep[1000];int col[1000];int n,son,ans;int flag[1000];int min(int a,int b){return a<b?a:b;}void dfs(int u,int fa,int t){    col[u]=1;    dep[u]=low[u]=t;    int tol=0;    int i,v;    for(i=0;i<list[u].size();i++)    {        v=list[u][i];        if(col[v]==2)   continue;        if(col[v]==0)        {            dfs(v,u,t+1);            tol++;            low[u]=min(low[u],low[v]);            if(u==1&&tol>1||low[v]>=dep[u]&&u!=1)            {                  flag[u]=1;            }        }        else if(col[v]==1&&v!=fa)        {            low[u]=min(low[u],dep[v]);        }    }}void init(){    son=0;ans=0;memset(flag,0,sizeof(flag));memset(dep,0,sizeof(dep));memset(col,0,sizeof(col));}int main(){    int i=0,j;int u,v;while(scanf("%d",&n),n){init();for(i=0;i<=n;i++)     list[i].clear();while(scanf("%d",&u),u)while(getchar()!='\n'){scanf("%d",&v);list[u].push_back(v);list[v].push_back(u);            }            dfs(1,-1,1);            for(i=1;i<=n;i++) ans+=flag[i];            printf("%d\n",ans);    }    return 0;}