Print nodes in a tree

       1
      / \
     2   3
    / \   \
   4   5   6
      /
     7
     
1234567

or 

1
23
456
7

Solution:

# recursive version
def print_tree(L):
        newL = []
        for n in L:
                print n
                if n.left <> None:
                        newL.append(n.left)
                if n.right <> None:
                        newL.append(n.right)
        print_tree(newL)
        print "\n"

Comment: we should use a queue structure and replace the loop with bottom() and the append with top() in order to avoid to have to create a list at each iteration

# iterative version
L = [n]
while (len(L)>0):
        newL = []
        for n in L:
                print n
                if n.left <> None:
                        newL.append(n.left)
                if n.right <> None:
                        newL.append(n.right)
        print "\n"
        L = newL