WA60分求助

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int maxn=1e5+10,maxm=6e5+10;
int n,m,q,col[maxn],dfn[maxn],low[maxn],tot=-1,head[maxn],cnt,tot2=-1;
int top,stk[maxn],us[maxm],vs[maxm],head2[maxn],anc[maxn][35],dep[maxn];
bool vis[maxn];
struct node{int v,nxt;}edge[maxm];
struct Nd{int v,nxt;}edge2[maxm];
void addedge(int u,int v){
    edge[++tot].v=v;
    edge[tot].nxt=head[u];
    head[u]=tot;
}
void addedge2(int u,int v){
    edge2[++tot2].v=v;
    edge2[tot2].nxt=head2[u];
    head2[u]=tot2;
}
void tarjan(int u,int lb){
    dfn[u]=low[u]=++cnt;
    stk[++top]=u;
    for(int i=head[u];i;i=edge[i].nxt){
        int v=edge[i].v;
        if(!dfn[v]){
            tarjan(v,i);
            low[u]=min(low[u],low[v]);
        }else if(i^(lb^1))low[u]=min(low[u],dfn[v]);
    }
    if(low[u]==dfn[u]){
        int curr;
        do{
            curr=stk[top--];
            col[curr]=u;
        }while(curr!=u);
    }
}
void dfs(int u,int fa){
    if(vis[u])return;
    vis[u]=1;
    dep[u]=dep[fa]+1;
    anc[u][0]=fa;
    for(int i=head2[u];i;i=edge2[i].nxt){
        int v=edge2[i].v;
        if(v==fa)continue;
        dfs(v,u);
    }
}
int LCA(int u,int v){
    if(dep[u]<dep[v])swap(u,v);
    for(int i=25;i>=0;i--)
        if(dep[anc[u][i]]>=dep[v])u=anc[u][i];
    if(u==v)return u;
    for(int i=25;i>=0;i--)if(anc[u][i]!=anc[v][i])u=anc[u][i],v=anc[v][i];
    return anc[u][0];
}
signed main(){
    ios::sync_with_stdio(0);
    cin.tie(0);cout.tie(0);
    cin>>n>>m;
    
    for(int i=1;i<=m;i++){
        cin>>us[i]>>vs[i];
        addedge(us[i],vs[i]);
        addedge(vs[i],us[i]);
    }
    int s=0;
    for(int i=1;i<=n;i++)if(!dfn[i])tarjan(i,0);
    for(int i=1;i<=m;i++)
        if(col[us[i]]!=col[vs[i]])
        {
            if(s==0)s=col[us[i]]; 
            addedge2(col[us[i]],col[vs[i]]);
            addedge2(col[vs[i]],col[us[i]]);
        }
    if(s!=0){
        dfs(s,0);
        for(int i=1;i<=20;++i)
                for(int u=1;u<=n;u++)
            		anc[u][i]=anc[anc[u][i-1]][i-1];
	
    }
    
    cin>>q;
    while(q--){
        int a,b;
        cin>>a>>b;
    if(s==0) cout<<"0\n"; 
    else 
        if(col[a]==col[b])cout<<"0\n";
        else {
            int lca=LCA(col[a],col[b]);
            cout<<dep[col[a]]+dep[col[b]]-2*dep[lca]<<endl;
        }
    }
    return 0;
}