DFS Traversal of a Tree Without using Recursion in Python

This is a Python program to perform depth-first search on a binary tree without using recursion.

Problem Description

The program creates a binary tree and presents a menu to the user to perform operations on the tree including depth-first search.

Problem Solution

1. Create a class Stack to implement a stack.
2. The class Stack will have methods is_empty, push and pop.
3. Create a class BinaryTree with instance variables key, left and right.
4. Define methods set_root, insert_left, insert_right, preorder_depth_first and search.
5. The method set_root takes a key as argument and sets the variable key equal to it.
6. The methods insert_left and insert_right insert a node as the left and right child respectively.
7. The method search returns a node with a specified key.
8. The method preorder_depth_first displays a preorder DFS traversal of the tree.
9. The preorder traversal is implemented using a stack to avoid recursion.

Program/Source Code

Here is the source code of a Python program to perform depth-first search on a binary tree without using recursion. The program output is shown below.

```class BinaryTree:
def __init__(self, key=None):
self.key = key
self.left = None
self.right = None

def set_root(self, key):
self.key = key

def insert_left(self, new_node):
self.left = new_node

def insert_right(self, new_node):
self.right = new_node

def search(self, key):
if self.key == key:
return self
if self.left is not None:
temp =  self.left.search(key)
if temp is not None:
return temp
if self.right is not None:
temp =  self.right.search(key)
return temp
return None

def preorder_depth_first(self):
s = Stack()
s.push(self)
while (not s.is_empty()):
node = s.pop()
print(node.key, end=' ')
if node.right is not None:
s.push(node.right)
if node.left is not None:
s.push(node.left)

class Stack:
def __init__(self):
self.items = []

def is_empty(self):
return self.items == []

def push(self, data):
self.items.append(data)

def pop(self):
return self.items.pop()

btree = BinaryTree()

print('Menu (this assumes no duplicate keys)')
print('insert <data> at root')
print('insert <data> left of <data>')
print('insert <data> right of <data>')
print('dfs')
print('quit')

while True:
do = input('What would you like to do? ').split()

operation = do[0].strip().lower()
if operation == 'insert':
data = int(do[1])
new_node = BinaryTree(data)
suboperation = do[2].strip().lower()
if suboperation == 'at':
btree = new_node
else:
position = do[4].strip().lower()
key = int(position)
ref_node = None
if btree is not None:
ref_node = btree.search(key)
if ref_node is None:
print('No such key.')
continue
if suboperation == 'left':
ref_node.insert_left(new_node)
elif suboperation == 'right':
ref_node.insert_right(new_node)

elif operation == 'dfs':
print('pre-order dfs traversal: ', end='')
if btree is not None:
btree.preorder_depth_first()
print()

elif operation == 'quit':
break```
Program Explanation

1. A variable is created to store the binary tree.
2. The user is presented with a menu to perform operations on the tree.
3. The corresponding methods are called to perform each operation.
4. The method preorder_depth_first is called to display a DFS pre-order traversal of the tree.

Runtime Test Cases
```Case 1:
Menu (this assumes no duplicate keys)
insert <data> at root
insert <data> left of <data>
insert <data> right of <data>
dfs
quit
What would you like to do? insert 1 at root
What would you like to do? insert 2 left of 1
What would you like to do? insert 3 right of 1
What would you like to do? insert 4 right of 2
What would you like to do? insert 5 left of 4
What would you like to do? dfs
pre-order dfs traversal: 1 2 4 5 3
What would you like to do? quit

Case 2:
Menu (this assumes no duplicate keys)
insert <data> at root
insert <data> left of <data>
insert <data> right of <data>
dfs
quit
What would you like to do? insert 3 at root
What would you like to do? insert 6 left of 3
What would you like to do? insert 7 right of 3
What would you like to do? insert 8 left of 7
What would you like to do? insert 10 right of 7
What would you like to do? dfs
pre-order dfs traversal: 3 6 7 8 10
What would you like to do? quit```

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