# UVa : 10938 – Flea circus

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 &lt;bits/stdc++.h&gt;

#define pii              pair &lt;int,int&gt;
#define pll              pair &lt;long long,long long&gt;
#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&lt;&lt;#x &quot; = &quot;&lt;&lt;(x)&lt;&lt;endl
#define VI               vector &lt;int&gt;
#define DBG              pf(&quot;Hi\n&quot;)
#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(&quot;%d&quot;,&amp;a)
#define sfl(a)           scanf(&quot;%lld&quot;,&amp;a)
#define sff(a,b)         scanf(&quot;%d %d&quot;,&amp;a,&amp;b)
#define sffl(a,b)        scanf(&quot;%lld %lld&quot;,&amp;a,&amp;b)
#define sfff(a,b,c)      scanf(&quot;%d %d %d&quot;,&amp;a,&amp;b,&amp;c)
#define sfffl(a,b,c)     scanf(&quot;%lld %lld %lld&quot;,&amp;a,&amp;b,&amp;c)
#define stlloop(v)       for(__typeof(v.begin()) it=v.begin();it!=v.end();it++)
#define loop(i,n)        for(int i=0;i&lt;n;i++)
#define loop1(i,n)       for(int i=1;i&lt;=n;i++)
#define REP(i,a,b)       for(int i=a;i&lt;b;i++)
#define RREP(i,a,b)      for(int i=a;i&gt;=b;i--)
#define TEST_CASE(t)     for(int z=1;z&lt;=t;z++)
#define PRINT_CASE       printf(&quot;Case %d: &quot;,z)
#define LINE_PRINT_CASE  printf(&quot;Case %d:\n&quot;,z)
#define CASE_PRINT       cout&lt;&lt;&quot;Case &quot;&lt;&lt;z&lt;&lt;&quot;: &quot;
#define all(a)           a.begin(),a.end()
#define intlim           2147483648
#define infinity         (1&lt;&lt;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
/*------------------------------------------------*/

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

#define mx 5005
int n,q;

vector&lt;int&gt;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&lt;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&lt;=n;i++)
{
sparse_par[i][0]=par[i];
sparse_dis[i][0]=1;
}
sparse_dis[1][0]=0;

for(int j=1; 1&lt;&lt;j&lt;=n;j++)
{
for(int i=1;i&lt;=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]&lt;=level[q]) swap(p,q);

int log=log2(level[p]);

int ret=0;

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

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

for(int i=log;i&gt;=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&lt;int&gt;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&lt;SZ(g2[u]);i++)
return dfs1(g2[u][i],cnt-1);
}

int main()
{

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

while(sf(n) &amp;&amp; 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(&quot;The fleas jump forever between %d and %d.\n&quot;,min(aa,bb),max(aa,bb));

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

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

}

return 0;
}

```