URAL 2034 : Caravans

#include <cstdio>
#include <vector>
#include <queue>
#include <cstring>
using namespace std;
const int maxn = 1e5 + 50;
#define max(x,y) ((x)>(y)?(x):(y))
#define min(x,y) ((x)<(y)?(x):(y))
vector<int>e[maxn];
int dp[maxn]  , d[maxn] , maxv[maxn], n , m , s , f , r , used[maxn] ;
queue<int>q;

void solve()
{
   memset(dp , 0 , sizeof(dp));memset(d,-1,sizeof(d));memset(maxv,-1,sizeof(maxv));memset(used,0,sizeof(used));
   d[f] = 0;
   q.push(f);
   while(!q.empty())
   {
         int x = q.front();q.pop();
         for(int i = 0 ; i < e[x].size() ; ++ i)
         {
             int v = e[x][i];
             if (d[v] == -1)
             {
                 d[v] = d[x] + 1;
                 q.push(v);
             }
         }
   }
   maxv[r] = 0;
   q.push(r);
   while(!q.empty())
   {
        int x = q.front();q.pop();
        for(int i = 0 ; i < e[x].size() ; ++ i)
        {
            int v = e[x][i];
            if (maxv[v] == -1)
            {
                maxv[v] = maxv[x] + 1;
                q.push(v);
            }
        }
   }
   dp[s] = maxv[s];used[s] = 1;
   q.push(s);
   while(!q.empty())
   {
         int x = q.front();q.pop();
         maxv[x] = min(maxv[x],dp[x]);
         for(int i = 0 ; i < e[x].size() ; ++ i)
         {
             int v = e[x][i];
             if (d[x] - d[v] == 1)
             {
                 dp[v] = max(dp[v],maxv[x]);
                 if (!used[v]) 
                 {
                     q.push(v);
                     used[v] = 1;
                 }
             }
         }
   }
   printf("%d\n",maxv[f]);
}

int main(int argc,char *argv[])
{
  scanf("%d%d",&n,&m);
  while(m--)
  {
       int u ,v ;
       scanf("%d%d",&u,&v);u--,v--;
       e[u].push_back(v);e[v].push_back(u);
  }
  scanf("%d%d%d",&s,&f,&r);s--,f--,r--;
  solve();
  return 0;
}