UVa : 10938 – Flea circus

Problem Link : http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=24&page=show_problem&problem=1879

Solution Idea: Find the LCA. Then find the middle node and middle+1 node on the path connection two nodes which are given in the query. Here I use an extra DFS to find the path on a reverse graph g2.


#include <bits/stdc++.h>

#define pii              pair <int,int>
#define pll              pair <long long,long long>
#define sc               scanf
#define pf               printf
#define Pi               2*acos(0.0)
#define ms(a,b)          memset(a, b, sizeof(a))
#define pb(a)            push_back(a)
#define MP               make_pair
#define db               double
#define ll               long long
#define EPS              10E-10
#define ff               first
#define ss               second
#define sqr(x)           (x)*(x)
#define D(x)             cout<<#x " = "<<(x)<<endl
#define VI               vector <int>
#define DBG              pf("Hi\n")
#define MOD              1000000007
#define CIN              ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define SZ(a)            (int)a.size()
#define sf(a)            scanf("%d",&a)
#define sfl(a)           scanf("%lld",&a)
#define sff(a,b)         scanf("%d %d",&a,&b)
#define sffl(a,b)        scanf("%lld %lld",&a,&b)
#define sfff(a,b,c)      scanf("%d %d %d",&a,&b,&c)
#define sfffl(a,b,c)     scanf("%lld %lld %lld",&a,&b,&c)
#define stlloop(v)       for(__typeof(v.begin()) it=v.begin();it!=v.end();it++)
#define loop(i,n)        for(int i=0;i<n;i++)
#define loop1(i,n)       for(int i=1;i<=n;i++)
#define REP(i,a,b)       for(int i=a;i<b;i++)
#define RREP(i,a,b)      for(int i=a;i>=b;i--)
#define TEST_CASE(t)     for(int z=1;z<=t;z++)
#define PRINT_CASE       printf("Case %d: ",z)
#define LINE_PRINT_CASE  printf("Case %d:\n",z)
#define CASE_PRINT       cout<<"Case "<<z<<": "
#define all(a)           a.begin(),a.end()
#define intlim           2147483648
#define infinity         (1<<28)
#define ull              unsigned long long
#define gcd(a, b)        __gcd(a, b)
#define lcm(a, b)        ((a)*((b)/gcd(a,b)))

using namespace std;


/*----------------------Graph Moves----------------*/
//const int fx[]={+1,-1,+0,+0};
//const int fy[]={+0,+0,+1,-1};
//const int fx[]={+0,+0,+1,-1,-1,+1,-1,+1};   // Kings Move
//const int fy[]={-1,+1,+0,+0,+1,+1,-1,-1};  // Kings Move
//const int fx[]={-2, -2, -1, -1,  1,  1,  2,  2};  // Knights Move
//const int fy[]={-1,  1, -2,  2, -2,  2, -1,  1}; // Knights Move
/*------------------------------------------------*/

/*-----------------------Bitmask------------------*/
//int Set(int N,int pos){return N=N | (1<<pos);}
//int reset(int N,int pos){return N= N & ~(1<<pos);}
//bool check(int N,int pos){return (bool)(N & (1<<pos));}
/*------------------------------------------------*/

#define mx 5005
int n,q;

vector<int>g1[mx],g2[mx];

int sparse_par[mx][14],sparse_dis[mx][14];

int level[mx],par[mx];

void dfs(int u, int cnt, int from)
{
    par[u]=from;
    level[u]=cnt;
    g2[u].pb(from);

    for(int i=0;i<SZ(g1[u]);i++)
    {
        int v=g1[u][i];
        if(v!=from)
        {
            dfs(v,cnt+1,u);
        }
    }
}


void build_talbe()
{
    for(int i=1;i<=n;i++)
    {
        sparse_par[i][0]=par[i];
        sparse_dis[i][0]=1;
    }
    sparse_dis[1][0]=0;

    for(int j=1; 1<<j<=n;j++)
    {
        for(int i=1;i<=n;i++)
        {
            sparse_par[i][j]=sparse_par[sparse_par[i][j-1]][j-1];
            sparse_dis[i][j]=sparse_dis[i][j-1]+sparse_dis[sparse_par[i][j-1]][j-1];
        }
    }
}

int LCA;

int query(int p, int q)
{
    if(level[p]<=level[q]) swap(p,q);

    int log=log2(level[p]);

    int ret=0;

    for(int i=log;i>=0;i--)
    {
        if(level[p]-(1<<i)>=level[q])
        {
            ret+=sparse_dis[p][i];
            p=sparse_par[p][i];
        }
    }


    if(p==q){LCA=p; return ret;}

    for(int i=log;i>=0;i--)
    {
        if(sparse_par[p][i]!=sparse_par[q][i])
        {
            ret+=sparse_dis[p][i];
            ret+=sparse_dis[q][i];
            p=sparse_par[p][i];
            q=sparse_par[q][i];
        }
    }
    ret+=2;
    LCA=par[p];
    return ret;
}


vector<int>temp;

int dfs1(int u, int cnt)
{
    if(u==LCA) return u;
    temp.pb(u);
    if(cnt==0) return u;
    for(int i=0;i<SZ(g2[u]);i++)
        return dfs1(g2[u][i],cnt-1);
}

int main()
{

//    freopen("in.txt","r",stdin);
//    freopen("out.txt","w",stdout);

    while(sf(n) && n)
    {
        loop(i,n-1)
        {
            int a,b;
            sff(a,b);
            g1[a].pb(b);
            g1[b].pb(a);
        }

        dfs(1,0,1);

        build_talbe();

        sf(q);

        while(q--)
        {
            int a,b;
            sff(a,b);

            int dis=query(a,b)+1;

            if(dis%2==0)
            {
                temp.clear();
                dfs1(a,n);
                temp.pb(LCA);
                int x=SZ(temp);
                dfs1(b,n);
                reverse(temp.begin()+x,temp.end());
                int aa=temp[(dis/2)-1];
                int bb=temp[dis/2];
                pf("The fleas jump forever between %d and %d.\n",min(aa,bb),max(aa,bb));

            }
            else
            {
                int p;
                if(level[a]>level[b])
                    p=a;
                else
                    p=b;
                pf("The fleas meet at %d.\n",dfs1(p,dis/2));
            }
        }

        for(int i=0;i<=n;i++)
        {
            g1[i].clear();
            g2[i].clear();
        }

    }


    return 0;
}


Advertisements