Python Program to Implement Depth First Search Traversal using Post Order

This is a Python program to perform depth-first post-order traversal on a tree.

Problem Description

The program creates a tree and presents a menu to the user to perform operations on the tree including post-order traversal.

Problem Solution

1. Create a class Tree with instance variables key and children.
2. Define methods set_root, add, postorder and search.
3. The method set_root takes a key as argument and sets the instance variable key equal to it.
4. The method add appends a node to the list children.
5. The method search returns a node with a specified key.
6. The method postorder displays a DFS post-order traversal of the tree.

Program/Source Code

Here is the source code of a Python program to perform depth-first post-order traversal on a tree. The program output is shown below.

class Tree:
    def __init__(self, data=None):
        self.key = data
        self.children = []
 
    def set_root(self, data):
        self.key = data
 
    def add(self, node):
        self.children.append(node)
 
    def search(self, key):
        if self.key == key:
            return self
        for child in self.children:
            temp = child.search(key)
            if temp is not None:
                return temp
        return None
 
    def postorder(self):
        for child in self.children:
            child.postorder()
        print(self.key, end=' ')
 
 
tree = None
 
print('Menu (this assumes no duplicate keys)')
print('add <data> at root')
print('add <data> below <data>')
print('dfs')
print('quit')
 
while True:
    do = input('What would you like to do? ').split()
 
    operation = do[0].strip().lower()
    if operation == 'add':
        data = int(do[1])
        new_node = Tree(data)
        suboperation = do[2].strip().lower() 
        if suboperation == 'at':
            tree = new_node
        elif suboperation == 'below':
            position = do[3].strip().lower()
            key = int(position)
            ref_node = None
            if tree is not None:
                ref_node = tree.search(key)
            if ref_node is None:
                print('No such key.')
                continue
            ref_node.add(new_node)
 
    elif operation == 'dfs':
        print('Post-order traversal: ', end='')
        tree.postorder()
        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 postorder is called on the tree to display a DFS post-order traversal.

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Runtime Test Cases
Case 1:
Menu (this assumes no duplicate keys)
add <data> at root
add <data> below <data>
dfs
quit
What would you like to do? add 1 at root
What would you like to do? add 2 below 1
What would you like to do? add 3 below 2
What would you like to do? add 4 below 2
What would you like to do? add 5 below 1
What would you like to do? add 6 below 1
What would you like to do? dfs
Post-order traversal: 3 4 2 5 6 1 
What would you like to do? quit
 
Case 2:
Menu (this assumes no duplicate keys)
add <data> at root
add <data> below <data>
dfs
quit
What would you like to do? add 3 at root
What would you like to do? add 5 below 3
What would you like to do? add 2 below 3
What would you like to do? add 1 below 5
What would you like to do? add 6 below 5
What would you like to do? dfs
Post-order traversal: 1 6 5 2 3 
What would you like to do? quit

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