UVA - 12487 Midnight Cowboy（LCA+思维）

#include<iostream>
#include<cmath>
#include<cstring>
#include<queue>
#include<vector>
#include<cstdio>
#include<algorithm>
#include<map>
#include<set>
#define rep(i,e) for(int i=0;i<(e);i++)
#define rep1(i,e) for(int i=1;i<=(e);i++)
#define repx(i,x,e) for(int i=(x);i<=(e);i++)
#define X first
#define Y second
#define PB push_back
#define MP make_pair
#define mset(var,val) memset(var,val,sizeof(var))
#define scd(a) scanf("%d",&a)
#define scdd(a,b) scanf("%d%d",&a,&b)
#define scddd(a,b,c) scanf("%d%d%d",&a,&b,&c)
#define pd(a) printf("%d\n",a)
#define scl(a) scanf("%lld",&a)
#define scll(a,b) scanf("%lld%lld",&a,&b)
#define sclll(a,b,c) scanf("%lld%lld%lld",&a,&b,&c)
#define IOS ios::sync_with_stdio(false);cin.tie(0)
using namespace std;
typedef long long ll;
template <class T>
void test(T a){cout<<a<<endl;}
template <class T,class T2>
void test(T a,T2 b){cout<<a<<" "<<b<<endl;}
template <class T,class T2,class T3>
void test(T a,T2 b,T3 c){cout<<a<<" "<<b<<" "<<c<<endl;}
const int inf = 0x3f3f3f3f;
const ll INF = 0x3f3f3f3f3f3f3f3fll;
const ll mod = 1e9+7;
int T;
void testcase(){
    printf("Case %d: ",++T);
}
const int MAXN = 500;
const int MAXM = 30;

struct Edge{
    int to,nxt;
}e[MAXN*2];
int head[MAXN],tot;
void addEdge(int u,int v){
    e[tot].to=v;
    e[tot].nxt=head[u];
    head[u]=tot++;
}
void init(){
    tot=0;
    mset(head,-1);
}
int fa[MAXN][20];
int deg[MAXN];
void BFS(int rt){
    queue<int> que;
    deg[rt]=0;
    fa[rt][0]=rt;
    que.push(rt);
    while(!que.empty()){
        int tmp = que.front();
        que.pop();
        for(int i=1;i<20;i++) fa[tmp][i]=fa[fa[tmp][i-1]][i-1];
        for(int i=head[tmp];~i;i=e[i].nxt){
            int v = e[i].to;
            if(v==fa[tmp][0]) continue;
            deg[v]=deg[tmp]+1;
            fa[v][0]=tmp;
            que.push(v);
        }
    }
}


int LCA(int u,int v){
    if(deg[u]>deg[v]) swap(u,v);
    int hu = deg[u],hv=deg[v];
    int tu = u,tv=v;
    for(int det=hv-hu,i=0;det;det>>=1,i++){
        if(det&1) tv=fa[tv][i];
    }
    if(tu==tv) return tu;
    for(int i=19;i>=0;i--){
        if(fa[tu][i]==fa[tv][i]){
            continue;
        }
        tu=fa[tu][i];
        tv=fa[tv][i];
    }
    return fa[tu][0];
}

int main() {
#ifdef LOCAL
    freopen("in.txt","r",stdin);
#endif // LOCAL
    int A,B,C,N;
    while(~scanf("%d%d%d%d",&N,&A,&B,&C)){
        init();
        int u,v;
        for(int i=1;i<N;i++){
            scdd(u,v);
            addEdge(u,v);
            addEdge(v,u);
        }
        BFS(A);
        int lca =LCA(B,C);
        int c=deg[C]-deg[lca];
        int b=deg[B]-deg[lca];
        double ans=0;
        if(c==0) ans=0;
        else if(b==0) ans=1;
        else ans=1.0*c/(c+b);
        printf("%.6f\n",ans);
    }
    return 0;
}