hdu 4612 Warm up 雙連通縮點+樹的直徑

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首先雙連通縮點建立新圖（順帶求原圖的總的橋數，其實由於原圖是一個強連通圖，所以橋就等於縮點後的邊）

此時得到的圖類似樹結構，對於新圖求一次直徑，也就是最長鏈。

我們建立的邊就一定是串連這條最長鏈的首尾，這樣就將原圖的橋減少了直徑個。

#include<iostream>#include<cstring>#include<cstdio>#include<vector>#include<algorithm>#include<map>using namespace std;#pragma comment(linker, "/STACK:102400000,102400000")#define maxn 200005#define maxm 2000005struct node{    int to,vis,next;}e[maxm],e2[maxm];int head[maxn],head2[maxn],en,en2;int belong[maxn],vis[maxn],dfn[maxn],low[maxn],cnt,bridge,col,stack[maxn],top;bool use[maxn];void add(int a,int b){    e[en].to=b;    e[en].vis=0;    e[en].next=head[a];    head[a]=en++;    e[en].to=a;    e[en].vis=0;    e[en].next=head[b];    head[b]=en++;}void add2(int a,int b){    e2[en2].to=b;    e2[en2].next=head2[a];    head2[a]=en2++;    e2[en2].to=a;    e2[en2].next=head2[b];    head2[b]=en2++;}void init(){    top=cnt=col=bridge=en2=en=0;    memset(vis,0,sizeof(vis));    memset(dfn,0,sizeof(dfn));    memset(low,0,sizeof(low));    memset(use,0,sizeof(use));    memset(head2,-1,sizeof(head2));    memset(head,-1,sizeof(head));}void Tarjan (int u){    int v;    vis[u] = 1;    dfn[u] = low[u] = ++cnt;    stack[top++] = u;    for (int i = head[u];i != -1;i = e[i].next)    {        v = e[i].to;        if (e[i].vis) continue;        e[i].vis = e[i^1].vis = 1;        if (vis[v] == 1)            low[u] = min(low[u],dfn[v]);        if (!vis[v])        {            Tarjan (v);            low[u] = min(low[u],low[v]);            if (low[v] > dfn[u])                bridge ++;        }    }    if (dfn[u] == low[u])    {        ++col;        do{            v = stack[--top];            vis[v] = 0;            belong[v] = col;        }while (u != v);    }}int len,st;void dfs(int now,int sum,int fa){    use[now]=1;    if(sum>len) {st=now;len=sum;}    for(int i=head2[now];~i;i=e2[i].next)    {        if(!use[e2[i].to]) dfs(e2[i].to,sum+1,now);    }}inline int ReadInt(){    char ch = getchar();    int data = 0;    while (ch < '0' || ch > '9')    {        ch = getchar();    }    do    {        data = data*10 + ch-'0';        ch = getchar();    }while (ch >= '0' && ch <= '9');        return data;}int main(){    int n,m,a,b;    while(scanf("%d%d",&n,&m)!=EOF)    {        if(!n&&!m) break;        init();        for(int i=1;i<=m;i++)        {            a=ReadInt();            b=ReadInt();            add(a,b);        }        Tarjan(1);        for(int i=1;i<=n;i++)            for(int j=head[i];~j;j=e[j].next)            {                int to=e[j].to;                if(belong[to]!=belong[i]) {add2(belong[to],belong[i]);}            }        len=0;dfs(1,0,-1);        memset(use,0,sizeof(use));        len=0;dfs(st,0,-1);        printf("%d\n",bridge-len);    }    return 0;}