No root tree transforms into a root tree

In a number of topics will encounter such a problem: undirected connectivity Graph G has n points, n-1 bar edge. Points are numbered from 1 to n sequentially .... is obviously a tree structure, but do not know the specific parent-child relationship, at this time need to transform a tree without root tree into a root tree, specifically explained as follows:

1. Tree storage: If the number of points is large, you need to use vector storage

vector<int> G[MAXN]; void Read_tree () {       int  u,v;       scanf ("%d",&N);         for (i=1; i<=n-1; i++) {             scanf ("%d%d",& u,&v);             G[u].push_back (v);             G[v].push_back (u);            }           }

　

2. Conversions: Conversions are done by a Dfs that fills the fa[in recursion]

 void  dfs (int  u,int  father) {//  OK tree with u as root, Father is U's father       int  d=g[u].size ();  for  (int  i= 0 ; I<d;i++) { int  v=g[u][i];  if  (v!=father) //  Prevent backwards search back, enter the dead loop  dfs (V,FA[V]=U); //  recursion + Assignment   

3. Small details: Initialize the time to make fa[root]=-1, meaning root without father, call is Dfs (ROOT,-1)

　　

No root tree transforms into a root tree