C++ :

#include<cstdio>
int l[1000],r[1000],f[1000];
int dfs(int n)
{
	if(n==f[n])
		return n;
	return dfs(f[n]);
}
void xianxu(int k)
{
	printf("%d ",k);
	if(l[k])
		xianxu(l[k]);
	if(r[k])
		xianxu(r[k]);
}
void zhongxu(int k)
{
	if(l[k])
		zhongxu(l[k]);
	printf("%d ",k);
	if(r[k])
		zhongxu(r[k]);
}
void houxu(int k)
{
	if(l[k])
		houxu(l[k]);
	if(r[k])
		houxu(r[k]);
	printf("%d ",k);
}
int main()
{
	int i,n,a,b,c;
	scanf("%d",&n);
	for(i=1;i<=n;i++)
		f[i]=i;
	for(i=1;i<=n;i++)
	{
		scanf("%d%d%d",&a,&b,&c);
		l[a]=b,r[a]=c;
		f[b]=f[c]=a;
	}
	int root=dfs(n);
	printf("%d\n",root);
	xianxu(root);
	printf("\n");
	zhongxu(root);
	printf("\n");
	houxu(root);
	printf("\n");
	return 0;
}

Pascal :

program p30;
type
  tree=record
    father:integer;
    left:integer;
    right:integer;
  end;
var
  a:array[1..1000] of tree;
  n,i,root,f,l,r,t:integer;
procedure first(t:integer);
  begin
    if t<>0 then
      begin
        write(t,' ');
        first(a[t].left);
        first(a[t].right);
      end;
  end;
procedure second(t:integer);
  begin
    if t<>0 then
      begin
        second(a[t].left);
        write(t,' ');
        second(a[t].right);
      end;
  end;
procedure third(t:integer);
  begin
    if t<>0 then
      begin
        third(a[t].left);
        third(a[t].right);
        write(t,' ');
      end;
  end;
begin
  readln(n);
  for i:=1 to n do
    begin
      readln(f,l,r);
      if l<>0 then
        begin
          a[l].father:=f;
          a[f].left:=l;
        end;
      if r<>0 then
        begin
          a[r].father:=f;
          a[f].right:=r;
        end;
    end;
  for i:=1 to n do
    if a[i].father=0 then
      root:=i;
  writeln(root);
  first(root);
  writeln;
  second(root);
  writeln;
  third(root);
end.