小A的最短路

题解：LCA算法模板

#include
#define mp make_pair
#define se second
#define fi first
using namespace std;

typedef long long ll;
//typedef pair pii;
//typedef pair pli;
//typedef pair pll;
typedef long double ld;

const int N=1e6+10;
const int MAXN=20010;
const int INF=0x3f3f3f3f;
const double eps=0.0000001;
const ll mod=998244353;
int head[N],to[N],nx[N],tot=1;//n个点。n-1条边
int dep[N];
int sz[N];
int fa[N][32];
int n;
void add(int u,int v){
    to[tot]=v;
    nx[tot]=head[u];
    head[u]=tot++;
}
void dfs(int u,int f,int step){
    dep[u]=step;
    fa[u][0]=f;
    sz[u]=1;
    for(int i=1;i<=21;i++){
        fa[u][i]=fa[fa[u][i-1]][i-1];
 
    }
    for(int i=head[u];i;i=nx[i]){
        int v=to[i];
        if(f==v)continue;
        dfs(v,u,step+1);
        sz[u]+=sz[v];
    }
}
int LCA(int u,int v){
    if(dep[u]<dep[v])swap(u,v);
    int d=dep[u]-dep[v];
    for(int i=0;(1<<i)<=d;i++){
        if((1<<i)&d){
            u=fa[u][i];
        }
    }
    if(u==v)return u;
    for(int i=21;i>=0;i--){
        if(fa[v][i]!=fa[u][i]){
            v=fa[v][i];
            u=fa[u][i];
        }
    }
    return fa[u][0];
}
int main(){
    int n; scanf("%d",&n);
    memset(head,-1,sizeof(head));
    for(int i = 1; i < n; ++i){
        int u,v;scanf("%d%d",&u,&v);
        add(u,v);
        add(v,u);
    }
    dfs(1,0,1);
    int a,b;scanf("%d%d",&a,&b);
    int q; scanf("%d",&q);
    while(q--){
        int u, v;
        scanf("%d%d",&u,&v);
        int lca = LCA(u,v);
        int ans = dep[u] +dep[v] - dep[lca]*2;
        int x = LCA(u,a);
        int y = LCA(b,v);
        ans = min(ans,dep[u] +dep[a] - dep[x]*2 + dep[b] +dep[v] - dep[y]*2);
        x = LCA(u,b);
        y = LCA(v,a);
        ans = min(ans,dep[u] +dep[b] - dep[x]*2 + dep[a] +dep[v] - dep[y]*2);
        printf("%d\n",ans);
    }
    return 0;
}