I was doing this leetcode question:(https://leetcode.com/problems/binary-tree-inorder-traversal/) in which I came up with this solution:
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def inorderTraversal(self, root: TreeNode) -> List[int]:
if root is None:
return None
result = []
if root.left is None and root.right is None:
result.append(root.val)
return result
return self.traverse(root,result)
def traverse(self,node,result):
if node is None:
return result
result = self.traverse(node.left,result)
result.append(node.val)
result = self.traverse(node.right,result)
return result
However I found out I actually don't need to store the results of recursion call in the variable and I can simply do this:
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def inorderTraversal(self, root: TreeNode) -> List[int]:
if root is None:
return None
result = []
if root.left is None and root.right is None:
result.append(root.val)
return result
return self.traverse(root,result)
def traverse(self,node,result):
if node is None:
return result
self.traverse(node.left,result)
result.append(node.val)
self.traverse(node.right,result)
return result
My understanding was that in each recursion call, we are passing a reference to the result variable, not copying the result variable, so what is happening is that when the recursion call gets to the left most node, it appends the value and returns to its parent node, and since we had pass by reference, the result variable in parent node already has the leftmost node added to it, so it just adds the parent node to it and keep continuing on the recursion.
Is my understanding correct or is there something else going on?
Thanks
Yes, your understanding is right.
Note: you are sharing the same code in both boxes.
Related
I'm learning about data structures and algorithms and I'm starting to learn about constructing linked lists from scratch in python. Right now I understand how they work and the components that go into making them (Nodes, data/address, Head/Tail, etc), but I'm having a really hard time wrapping my brain around how they function when constructing them in python. Like I have working code to make them in python here but I don't get the logic behind how they operate with classes. For example, I'm confused in my addLast-function on how the node variable(node = Node(value)) connects to the Node class.
class Node:
def __init__(self, value, next=None):
self.value = value
self.next = next
class LinkedList:
def __init__(self):
self.head = None
self.tail = None
def addLast(self, value):
node = Node(value)
if self.head == None:
self.head = node
self.tail = node
else:
self.tail.next = node
self.tail = node
class Node:
def __init__(self, value, next=None):
self.value = value
# NODE POINTS TO THE NEXT ELEMENT IF PROVIDED ELSE NONE
self.next = next
class LinkedList:
def __init__(self):
# INIT AN EMPTY LINKED LIST
self.head = None
self.tail = None
def addLast(self, value):
# CREATE A NODE OBJECT WITH VALUE 'value' WHICH POINTS TO NOTHING (because it's the end of the linked list)
node = Node(value)
# IF NO HEAD IT BECOMES THE HEAD AND THE TAIL
if self.head == None:
self.head = node
self.tail = node
else:
# ADD THE NODE TO THE END (tail) OF THE LINKED LIST
self.tail.next = node
self.tail = node
# Create an empty linked_list
head = Linked_list()
# Add at the end a node with the value 1
head.addLast(1)
Hope it's clearer for you, ask questions if needed
I think this may help you understand what the code actually do hedind the scenes.
You can paste the following code to the link and click the "Visualize Execution" button. It will visualize all details step by step.
Good Luck!
class Node:
def __init__(self, value, next=None):
self.value = value
self.next = next
class LinkedList:
def __init__(self):
self.head = None
self.tail = None
def addLast(self, value):
node = Node(value)
if self.head == None:
self.head = node
self.tail = node
else:
self.tail.next = node
self.tail = node
head = LinkedList()
head.addLast(1)
head.addLast(2)
I am taking Data Structures and Algorithm course from Jovian. Currently on Binary Tress, but I am stuck on one question where we need to calculate no. of nodes in O(1) time.
Firstly here's how the final class TreeMap looks like:
class TreeMap:
def __init__(self):
self.root = None
def __setitem__(self, key, value):
node = find(self.root, key)
if not node:
self.root = insert(self.root, key, value)
self.root = balance_tree(self.root)
else:
update(self.root, key, value)
def __getitem__(self, key):
node = find(self.root, key)
return node.value if node else None
def __iter__(self):
return (x for x in list_all(self.root))
def __len__(self):
return size(self.root)
def display(self):
return display_keys(self.root):
Currently, it's calculating no. of nodes with the recursion method. I think we just need a counter and increment every time a node is created and we also have a hint to modify the BSTNode class. So this is how I did it:
class BSTNode:
counter = 0
def __init__(self, key, value=None):
self.key = key
self.value = value
self.left = None
self.right = None
self.parent = None
BSTNode.counter += 1
But I don't know how do I implement or use the counter in __len__ function in class TreeMap. Any help would be much appreciated.
Here is the link to Jovian Lesson: https://jovian.ai/learn/data-structures-and-algorithms-in-python/lesson/lesson-2-binary-search-trees-traversals-and-balancing
Python 3 - I am new to coding and am finding recursion difficult. I'm making a linked list class with recursive methods for adding and removing items from the list. Right now, I am unable to remove an item if it happens to be the first item in my list. I wrote some alternative code which could remove the first item from the list if I included another parameter (previous) and another base case, but then I could only remove the first item and spent way too long trying to figure out why so I scrapped that entirely. I would appreciate a hint!
Also, I am already aware that I have getters and am not using them properly.
class Node:
"""
Represents a node in a linked list
"""
def __init__(self, data):
self._data = data
self._next = None
def get_data(self):
"""getter method for data in Node class"""
return self._data
def get_next(self):
"""getter method for next in Node class"""
return self._next
class LinkedList:
"""
A linked list implementation of the List ADT
"""
def __init__(self):
self._head = None
def get_head(self):
"""getter function for head of list"""
return self._head
def add(self, val):
""" Adds a node containing val to the linked list - helper function"""
self._head = self.recursive_add(self._head, val)
def recursive_add(self, node1, val):
""" Adds a node containing val to the linked list """
if node1 is None:
return Node(val)
else:
node1._next = self.recursive_add(node1._next, val)
return node1
def remove(self, val):
"""removed the node containing val from the linked list - helper function"""
self.recursive_remove(self._head, val)
def recursive_remove(self, node1, val):
"""
Removes the node containing val from the linked list
"""
if node1 is None:
return node1
elif node1._data == val:
return node1._next
else:
node1._next = self.recursive_remove(node1._next, val)
return node1
def main():
my_list = LinkedList()
my_list.add(13)
my_list.add(9)
my_list.add(5)
my_list.remove(9)
if __name__ == '__main__':
main()
def remove(self, val):
"""removed the node containing val from the linked list - helper function"""
if self._head and self._head._data == val:
self._head = self._head._next
return
self.recursive_remove(self._head, val)
if its at the start, the head needs to be changed.
In remove you call recursive_remove, but ignore its return value. You should use it as the (potentially different) _head reference, must like is done in the recursive method itself, where you have:
node1._next = self.recursive_remove(node1._next, val)
# ^ │
# └───────────────────────────────────┘
Note how node1._next is passed as argument, and the method's return value is the (potentially different) reference that node1._next should end up with. The same pattern should be applied in the initial call in remove:
def remove(self, val):
self._head = self.recursive_remove(self._head, val)
# ^ │
# └──────────────────────────────────┘
NB: the same pattern is used in add, where you do it correctly.
I'm a bit confused regarding local and global variables referenced outside a function scope. For one problem, I had this code and it worked:
def leafSimilar(self, root1: TreeNode, root2: TreeNode) -> bool:
l1 = []
l2 = []
def traverseLeaf(root: TreeNode, sequence: list):
if not root: return
elif not root.left and not root.right: sequence.append(root.val)
else:
traverseLeaf(root.left, sequence)
traverseLeaf(root.right, sequence)
traverseLeaf(root1, l1)
traverseLeaf(root2, l2)
return l1 == l2
You can see I reference the list object outside the function scope. However, if I try this:
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def convertBST(self, root: TreeNode) -> TreeNode:
total = 0
def rightOrderTraversal(node: TreeNode):
if not node: return
elif not node.left and not node.right:
total += node.val
node.val = total
else:
rightOrderTraversal(node.right)
total += node.val
node.val = total
rightOrderTraversal(node.left)
rightOrderTraversal(root)
return root
It does not work, and the error says I reference total as a local variable when calling rightOrderTraversal. What kind of variables does Python create local references for? From these examples, it seems like Python references lists outside the function scope but not ints.
I also tried to make total global as a way to solve the issue, but that didn't work.
If you want to use a variable that is visible to any methods/sub functions within an object instance, you use the "self" to reference to the object instance, eg:
class Solution:
def my_method(self, total):
self.total = total
def my_function(total):
self.total += total
my_function(total)
s = Solution()
s.my_method(1)
print(s.total)
I am very new to python and need some help with instantiating an object. The python interpreter is giving me trouble when instantiating an object of a class I defined. There are two classes, BTNode and BST (which are stored in files bst_node.py and bst.py respectively):
# file: bst_node.py
class BTNode:
"""a binary search tree node implementation"""
def ___init___(self, value):
self.value = value
self.left is None
self.right is None
self.parent is None
def ___init___(self, value, left, right, parent):
"""set the parameters to corresponding class members"""
self.value = value
self.left = left
self.right = right
self.parent = parent
def is_leaf(self):
"""check whether this node is a leaf"""
if self.left.value is None and self.right.value is None:
return True
return False
# file: bst.py
from bst_node import *
class BST:
"""a binary search tree implementation"""
def ___init___(self, value):
self.root = BTNode(value)
def insert(self, curRoot, newValue):
if curRoot.is_leaf():
if newValue < curRoot.value:
newNode = BTNode(newValue, None, None, curRoot)
curRoot.left = newNode
else:
newNode = BTNode(newValue, None, None, curRoot)
curRoot.right = newNode
else:
if newValue < curRoot.value:
self.insert(curRoot.left, newValue)
else:
self.insert(curRoot.right, newValue)
So, in the interpreter I do:
import bst as b
t1 = b.BST(8)
and I get an error which says that this constructor takes no arguments
The constructor clearly takes an argument value so what is going wrong here? How can I fix this error?
Thanks, all help is greatly appreciated!
The first issue is that you called your functions ___init___ instead of __init__. All of the 'special methods' use two underscores.
A second issue in this code is that in BTNode you redefined __init__. You can't overload functions in python. When you reclare __init__ you effectively deleted the first constructor.
A third issue is your usage of is. is is an operator that checks whether two objects are exactly the same and returns True or False. In the constructor, you have a few self.left is None is examining the value of self.left (which wasn't declared yet), and examining whether or not it is None. To set it, use = as follows:self.left = None
To fix the second and third issue you should use default argument values. For example:
def __init__(self, value, left=None, right=None, parent=None):
In addition to the number of underscores problem, you should replace
def ___init___(self, value):
self.value = value
self.left is None
self.right is None
self.parent is None
def ___init___(self, value, left, right, parent):
"""set the parameters to corresponding class members"""
self.value = value
self.left = left
self.right = right
self.parent = parent
with
def __init__(self, value, left=None, right=None, parent=None):
"""set the parameters to corresponding class members"""
self.value = value
self.left = left
self.right = right
self.parent = parent
Because as #Moshe points out, you can't overload functions, you should use default arguments insted.
Changing ___init___ to __init__ should fix it. (2 underscores vs 3)