The Truth of Sisyphus
  • Introduction
  • Deep Learning
    • Basics
      • Hinge Loss
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      • Multi-Class and Cross Entropy Loss
      • Batch Norm and other Normalizations
      • Optimization
      • Optimization Functions
      • Convolution im2col
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        • Derivatives of Softmax
        • A Smooth (differentiable) Max Function
      • Model Ensemble
      • Layers Python Implementation
    • Classification
      • Mobile friendly networks
      • Non-local Neural Networks
      • Squeeze-and-Excitation Networks
      • Further Attention Utilization -- Efficience & Segmentation
      • Group Norm
      • ShuffleNet V2
    • Segmentation
      • Several Instance Segmentation
      • A Peek at Semantic Segmentation
      • Design Choices for Mobile Friendly Deep Learning Models, Semantic Segmentation
      • Efficient Video Object Segmentation via Network Modulation
      • BiSeNet
      • DeepLabV3+
    • Detection
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      • IoU-Net
      • Why smooth L1 is popular in BBox Regression
      • MTCNN-NCNN
      • DetNet
      • SSD Illustration
    • RNN Related
      • GRU vs LSTM
      • BERT
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      • DRL for optimal execution of profolio transaction
    • Multi-task
      • Multi-task Overview
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    • Neural Network Interpretation
      • Neuron Visualization
    • Deep Learning Frameworks
      • How does Caffe work
      • [Gluon] When to use (Hybrid)Sequential and (Hybrid)Block
      • Gluon Hybrid Intro
      • Gluon HybridBlocks Walk-Through
      • A quick tour of Torch internals
      • NCHW / NHWC in Pytorch
      • Static & Dynamic Computation Graph
    • Converting Between DL Frameworks
      • Things To Be Considered When Doing Model Converting
      • Caffe to TensorFlow
    • Computation Graph Optimization
      • Two ways of TensorRT to optimize Neural Network Computation Graph
      • Customized Caffe Memory Optimization
      • NCNN Memory Optimization
      • Symbolic Programs Advantages: More Efficient, Reuse Intermediate Memory, Operation Folding
    • Deep Learning Debug
      • Problems caused by dead ReLU
      • Loss jumps to 87.3365
      • Common Causes of NANs During Training
    • Deployment
      • Efficient Convolution Operation
      • Quantization
    • What I read recently
      • Know Google the Paper Way
      • ECCV 2018
      • Neural Machine Translation
      • Street View OCR Extraction System
      • Teaching Machines to Draw
      • Pixel to Graph
      • Burst Image Deblurring
      • Material for Masses
      • Learning to Separate Object Sounds by Watching Unlabeled Video
    • Papers / Posts to be read
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  • Machine Learning
    • Classification
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    • Bayesian Example
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    • Recommendation
    • Essentials of Machine Learning
    • Linear Regression
    • Logistic Regression
      • Logistic Function
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    • MLE vs MAP
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    • Conclusion of Machine Learning
  • Python notes
    • Python _ or __ underscores usage
    • Python Multiprocess and Threading Differences
    • Heapq vs. Q.PriorityQueue
    • Python decorator
    • Understanding Python super()
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    • Is Python List a Linked List or Array
    • What is the "u" in u'Hello world'
    • Python "self"
    • Python object and class
    • Python Class' Instance method, Class method, and Static Methods Demystified
    • Python WTF
    • Python find first value index in a list: [list].index(val)
    • Sort tuples, and lambda usecase
    • Reverse order of range()
    • Python check list is empty
    • Python get ASCII value from character
    • An A-Z of useful Python tricks
    • Python nested function variable scope
    • Python reverse a list
    • Python priority queue -- heapq
  • C++ Notes
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  • Linux & Productivity
    • Jupyter Notebook on Remote Server
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  • Leetcode Notes
    • Array
      • 11. Container With Most Water
      • 35. Search Insert Position
    • Linked List
      • Difference between Linked List and Array
      • Linked List Insert
      • Design of Linked List
      • Two Pointers
        • 141. Linked List Cycle
        • 142. Linked List Cycle II
        • 160. Intersection of two Linked List
        • 19. Remove N-th node from the end of linked list
      • 206. Reverse Linked List
      • 203. Remove Linked List Elements
      • 328. Odd Even Linked List
      • 234. Palindrome Linked List
      • 21. Merge Two Sorted Lists
      • 430. Flatten a Multilevel Doubly Linked List
      • 430. Flatten a Multilevel Doubly Linked List
      • 708. Insert into a Cyclic Sorted List
      • 138. Copy List with Random Pointer
      • 61. Rotate List
    • Binary Tree
      • 144. Binary Tree Preorder Traversal
      • 94. Binary Tree Iterative In-order Traverse
    • Binary Search Tree
      • 98. Validate Binary Search Tree
      • 285. Inorder Successor in BST
      • 173. Binary Search Tree Iterator
      • 700. Search in a Binary Search Tree
      • 450. Delete Node in a BST
      • 701. Insert into a Binary Search Tree
      • Kth Largest Element in a Stream
      • Lowest Common Ancestor of a BST
      • Contain Duplicate III
      • Balanced BST
      • Convert Sorted Array to Binary Search Tree
    • Dynamic Programming
      • 198. House Robber
      • House Robber II
      • Unique Path
      • Unique Path II
      • Best time to buy and sell
      • Partition equal subset sum
      • Target Sum
      • Burst Ballons
    • DFS
      • Clone Graph
      • General Introduction
      • Array & String
      • Sliding Window
  • Quotes
    • Concert Violinist Joke
    • 船 Ship
    • What I cannot create, I do not understand
    • Set your course by the stars
    • To-do list
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  1. Leetcode Notes
  2. Binary Search Tree

450. Delete Node in a BST

Given a root node reference of a BST and a key, delete the node with the given key in the BST. Return the root node reference (possibly updated) of the BST.

Basically, the deletion can be divided into two stages:

  1. Search for a node to remove.

  2. If the node is found, delete the node.

Note: Time complexity should be O(height of tree).

Example:

root = [5,3,6,2,4,null,7]
key = 3

    5
   / \
  3   6
 / \   \
2   4   7

Given key to delete is 3. So we find the node with value 3 and delete it.

One valid answer is [5,4,6,2,null,null,7], shown in the following BST.

    5
   / \
  4   6
 /     \
2       7

Another valid answer is [5,2,6,null,4,null,7].

    5
   / \
  2   6
   \   \
    4   7

Deletion is more complicated than the two operations we mentioned before. There are also many different strategies for deletion. We are going to introduce one of them which minimizes the changes. Our solution is to replace the target node with a proper child. According to the number of its children, we should consider three different cases:1. If the target node has no child, we can simply remove the node. 2. If the target node has one child, we can use its child to replace itself. 3. If the target node has two children, replace the node with its in-order successor or predecessor node and delete that node.

Here are examples of different cases to help you understand this strategy.

By understanding the strategy above, you should be able to implement deletion function on your own. We have done an exercise about finding the inorder successor in a BST in the previous section. The solution for that question might help you implement the deletion function.

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):    
	def deleteNode(self, root, key):
		if not root:  # for the first root, if it is None, return None.
			return None

		# Modify either root, root.left or root.right and return the modified root
		if key < root.val:
			root.left = self.deleteNode(root.left, key) # neet to reassign the returned root to the proper child
		elif key > root.val:
			root.right = self.deleteNode(root.right, key) # neet to reassign the returned root to the proper child
		else: # found the target root
			if not root.left and not root.right: # target root has no children
				return None
			elif not root.left:  # target root has RIGHT child, replace root with the RIGHT child
				return root.right
			elif not root.right:  # target root has LEFT child, replace root with the LEFT child
				return root.left
			else:   # When root has both left and right children
				# Find the successor, replace root.val with successor.val, deleteNode(successor).
				# In this way, we don't need to know the parent of target root to delete the target root.
				successor = self.findSuccessorInRightChild(root) # Since currently root mush have left and right child, we only need to find the successor in the RIGHT child branch. This would be easier compared to maintain a stack of successors, which is sepcifically designed to find both the right child successor and successor in the other part of the tree.
				root.val = successor.val # replace root value with successor value will help us not thinking to much about the parent root of the target root / successor(since successor also need to be deleted and it will be the target root for the next recursive loop).
				
				'''
				This is WROING!!!! 
				We should modify root / root.left / root.right and return root, not return deleteNode(succesor, successor.val)
				Only return None when the root should be None (if not root.left and not root.right: return None)
				'''
				# return self.deleteNode(, successor.val) # recursively delete 
				'''
				Wrong anwser ends
				'''
				root.right = self.deleteNode(root.right, successor.val)

		return root


	def findSuccessorInRightChild(self, root):        
		cur = root.right        
		while(cur.left):            
			cur = cur.left        
		return cur
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