Don't always declare global variable in C/C++

If you always declare global variable instead of local,
Once you using recursive call, you may get run time error.
See

SPOJ Problem 1436: Is it a tree – PT07Y

if you move

list<int>::iterator it;

 from local variable to global variable, you will get run time error 

by input following:

3 2

1 3

3 2