Question

I am trying to write a code, which, given a Binary Search Tree root and a level, prints out the elements of the tree at that level. This is working fine:

def myprint(root,level):
    if root:
        if not level:
            print root.data,
        else:
            myprint(root.left,level-1)
            myprint(root.right,level-1)

However, when I try to tweak it to print the elements at a level in reverse order, it dosn’t work. For the following tree:

                         26
                    /          \
                   13          39
                  /  \        /  \
                 6    19     32   51
                / \   / \    / \  / \
               4   8  14    31 33   68
                        \
                         17

if I want to output the elements at level 3 (level of root is 0) from right to left, the output should be 68 33 31 14 8 4. The code above does the reverse correctly, that is, prints out 4 8 14 31 33 68. But the below code doesn’t print the reverse order correctly, and prints out 31 33 68 4 8 14 instead:

def revprint(root,level):
    if root:
        if not level:
            print root.data,
        else:
            myprint(root.right,level-1)
            myprint(root.left,level-1)

Can anybody spot the error, and tell me how to rectify it? The code for initializing the tree is as follows:

class tree:
    def __init__(self,data):
        self.data = data
        self.successor,self.left,self.right = None,None,None
    def push(self,data):
        root = self
        while root:
            oldroot = root
            if root.data > data:
                root = root.left
            elif root.data < data:
                root = root.right
        if data > oldroot.data:
            oldroot.right = tree(data)
        else:
            oldroot.left = tree(data)

a = tree(26)
for x in [13,39,6,19,4,8,5,10,9,14,17,15,32,51,68,31,33,36,34]:
a.push(x)

Answer 1

You are calling myprint in revprint. The fixed version of revprint:

def revprint(root,level):
    if root:
        if not level:
            print root.data,
        else:
            revprint(root.right,level-1)
            revprint(root.left,level-1)