POJ3694 Network(Tarjan双联通分图 LCA 桥)

#include<iostream>
#include<cstdio>
#include<string>
#include<cstring>
#include<cstdlib>
#include<queue>
#include<vector>
#include<algorithm>
const int INF = 0x3F3F3F3F;
using namespace std;
typedef long long LL;

const int N=100008,M=400008;
int head[N],tot,n,m,dfn[N],deep;
int bridge;
int fa[N], cnt[N], pre[N];
struct Node{
    int to,next;
}e[M];
void init(){
    memset(head, -1, sizeof(head));
    tot = 0;
    bridge = 0;
    for(int i = 1; i <= n; i++){
        fa[i] = i;
        cnt[i] = 1;
    }
}

int Find(int x){
    while(x != fa[x]){
        x = fa[x];
    }
    return x;
}
bool Union(int x, int y){
    int fx = Find(x);
    int fy = Find(y);
    if(fx!=fy){
    if(cnt[fx]>cnt[fy]){
            fa[fy]=fx;
            cnt[fx]+=cnt[fy];
        }else{
            fa[fx]=fy;
            cnt[fy]+=cnt[fx];
        }
        return true;
    }else{
        return false;
    }
}
inline void add(int u, int to){
    e[tot].to=to;
    e[tot].next=head[u];
    head[u]=tot++;
}
int dfs(int u, int fa){
    int lowu = dfn[u] = ++deep;//lowu为u及其后代所能到达的最远祖先
    for(int i=head[u];~i;i=e[i].next){
        int v = e[i].to;
        if(!dfn[v]){//树叶边，u到v且v未被访问
            pre[v] = u;
            int lowv = dfs(v, u);
            lowu = min(lowu, lowv);
            if(lowv > dfn[u]){
                 bridge++;
            }else{
                Union(u, v);
            }
        }else if(dfn[v] < dfn[u] && v != fa){
            lowu = min(lowu, dfn[v]);//后向边
        }
    }
    return lowu;
}

void tarjan(){
    memset(dfn, 0, sizeof(dfn));
    deep=0;
    for(int i=1;i<=n;i++){
        if(!dfn[i]){
            pre[i] = i;
            dfs(i,-1);
        }
    }
}

int LCA(int u, int v){
    if(Find(u) == Find(v)){
        return bridge;
    }
    if(dfn[u] > dfn[v]){
        swap(u, v);
    }
    while(dfn[u] < dfn[v]){
        if(Union(v, pre[v])){
            bridge--;
        }
        v = pre[v];
    }
    while(u != v){
        if(Union(u, pre[u])){
            bridge--;
        }
        u = pre[u];
    }
    return bridge;
}

int main(){
    int t = 0;
    while(~scanf("%d %d", &n, &m) && (n || m)){
        init();
        int a, b, q;
        while(m--){
            scanf("%d %d", &a, &b);
            add(a, b);
            add(b, a);
        }
        tarjan();
        scanf("%d", &q);
        printf("Case %d:\n",++t);
        while(q--){
            scanf("%d %d", &a, &b);
            printf("%d\n", LCA(a, b));
        }
        printf("\n");
    }
    return 0;
}