AcWing398 交通路径查询（点双）

无向图点双建立园方树，并且记录边在哪个点双之内

#include<bits/stdc++.h>
using namespace std;
typedef pair<int,int> pll;
const int N=3e5+10;
int h[N],e[N],ne[N],idx;
int dfn[N],ins[N],low[N],w[N],times;
int depth[N],fa[N],ans[N][25],fa_e[N];
int vcc[N];
stack<int> st,st_e;
vector<int> g[N];
int n,m;
int cnt;
void add(int a,int b,int c){
    e[idx]=b,ne[idx]=h[a],w[idx]=c,h[a]=idx++;
}
void dfs(int u){
    ins[u]=1;
    dfn[u]=low[u]=++times;
    int i;
    st.push(u);
    for(int i=h[u];i!=-1;i=ne[i]){
        int v=e[i],id=w[i];
        if(!dfn[v]){
            fa[v] = u; fa_e[v] = id;
            st_e.push(id);
            dfs(v);

            low[u] = min(low[u], low[v]);
            //if(dfn[u] < low[v]) isce[v] = 1;

            if(dfn[u] <= low[v]){
                ++cnt;
                while(1){
                    auto x = st.top();
                    g[cnt].push_back(x);
                    g[x].push_back(cnt);
                    ins[x] = 0; st.pop();
                    if(x == v) break;
                }
                g[cnt].push_back(u);
                g[u].push_back(cnt);

                while(1){
                    id = st_e.top(); st_e.pop();
                    vcc[id] = cnt;
                    if(id==fa_e[v]) break;
                }
            }
        }
        else if(v != fa[u]){
            low[u] = min(low[u], dfn[v]);
            if(dfn[v] < dfn[u]) st_e.push(id);
        }
    }
}
int vis[N];
void dfs2(int u){
    vis[u] = 1;
    for(int j=1;j<=20;j++){
        //if(depth[u] <= (1<<j)) 
            //break;
        ans[u][j] = ans[ans[u][j-1]][j-1];
    }

    int v;
    for(int k=0;k<g[u].size();k++){
        v = g[u][k];
        if(vis[v]) continue;
        ans[v][0] = u;
        depth[v] = depth[u] + 1;
        dfs2(v);
    }
}
int lca(int a,int b){
    if(depth[a]<depth[b])
    swap(a,b);
    int i;
    for(i=20;i>=0;i--){
        if(depth[ans[a][i]]>=depth[b]){
            a=ans[a][i];
        }
    }
    if(a==b)
    return a;
    for(i=20;i>=0;i--){
        if(ans[a][i]!=ans[b][i]){
            a=ans[a][i];
            b=ans[b][i];
        }
    }
    return ans[a][0];
}
int main(){

    while(1){
        scanf("%d%d", &n, &m);
        if(!m && !n) break;

        memset(fa, 0, sizeof(fa));
        memset(ans, 0, sizeof(ans));
        memset(dfn, 0, sizeof(dfn));
        memset(low, 0, sizeof(low));
        memset(depth, 0, sizeof(depth));
        //memset(inst, 0, sizeof(inst));
        memset(vis, 0, sizeof(vis));
        //memset(vdcc, 0, sizeof(vdcc));
        memset(h,-1,sizeof h);
        times=0;
        cnt = n;
        int u,v;
        for(int i=1;i<=m;i++){
            scanf("%d%d", &u, &v);
            add(u,v,i);
            add(v,u,i);
        }
        for(int i=1;i<=n;i++){
            if(!dfn[i]){
                dfs(i);
                depth[i] = 1;
                dfs2(i);
            }
        }
        int  Q;
        scanf("%d",&Q);
        int x, y;
        while(Q--){
            scanf("%d%d", &x, &y);
            x = vcc[x]; y = vcc[y];
            int z = lca(x,y);
            printf("%d
", (depth[x] + depth[y] - 2*depth[z])/2);
        }
        for(int i=1;i<=cnt;i++) g[i].clear(), g[i].clear();
    }
    return 0;
}