NoSQL vs SQL

NoSQL databases provide flexible data models ideal for diverse data structures and scalability.

1. Key-Value: Simple, uses key-value pairs (e.g., Redis).
2. Document: Stores data in JSON/BSON documents (e.g., MongoDB).
3. Graph: Manages complex relationships with nodes and edges (e.g., Neo4j).
4. Column Store: Optimized for analytics, organizes data by columns (e.g., Cassandra).

SQL databases, like RDBMS and OLAP, provide structured, relational storage for traditional and analytical needs

1. RDBMS: Traditional relational databases with tables (e.g., PostgreSQL & MySQL).
2. OLAP: Designed for complex analysis and multidimensional data (e.g., SQL Server Analysis Services).

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DSA (Data Structures and Algorithms) Essential Topics for Interviews

1️⃣ Arrays and Strings

Basic operations (insert, delete, update)

Two-pointer technique

Sliding window

Prefix sum

Kadane’s algorithm

Subarray problems


2️⃣ Linked List

Singly & Doubly Linked List

Reverse a linked list

Detect loop (Floyd’s Cycle)

Merge two sorted lists

Intersection of linked lists


3️⃣ Stack & Queue

Stack using array or linked list

Queue and Circular Queue

Monotonic Stack/Queue

LRU Cache (LinkedHashMap/Deque)

Infix to Postfix conversion


4️⃣ Hashing

HashMap, HashSet

Frequency counting

Two Sum problem

Group Anagrams

Longest Consecutive Sequence


5️⃣ Recursion & Backtracking

Base cases and recursive calls

Subsets, permutations

N-Queens problem

Sudoku solver

Word search


6️⃣ Trees & Binary Trees

Traversals (Inorder, Preorder, Postorder)

Height and Diameter

Balanced Binary Tree

Lowest Common Ancestor (LCA)

Serialize & Deserialize Tree


7️⃣ Binary Search Trees (BST)

Search, Insert, Delete

Validate BST

Kth smallest/largest element

Convert BST to DLL


8️⃣ Heaps & Priority Queues

Min Heap / Max Heap

Heapify

Top K elements

Merge K sorted lists

Median in a stream


9️⃣ Graphs

Representations (adjacency list/matrix)

DFS, BFS

Cycle detection (directed & undirected)

Topological Sort

Dijkstra’s & Bellman-Ford algorithm

Union-Find (Disjoint Set)


10️⃣ Dynamic Programming (DP)

0/1 Knapsack

Longest Common Subsequence

Matrix Chain Multiplication

DP on subsequences

Memoization vs Tabulation


11️⃣ Greedy Algorithms

Activity selection

Huffman coding

Fractional knapsack

Job scheduling


12️⃣ Tries

Insert and search a word

Word search

Auto-complete feature


13️⃣ Bit Manipulation

XOR, AND, OR basics

Check if power of 2

Single Number problem

Count set bits

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Topic: Data Structures – Trees – Top 15 Interview Questions with Answers

---

### 1. What is a tree data structure?

A hierarchical structure with nodes connected by edges, having a root node and child nodes with no cycles.

---

### 2. What is the difference between binary tree and binary search tree (BST)?

A binary tree allows up to two children per node; BST maintains order where left child < node < right child.

---

### 3. What are the types of binary trees?

Full, perfect, complete, skewed (left/right), and balanced binary trees.

---

### 4. Explain tree traversal methods.

Inorder (LNR), Preorder (NLR), Postorder (LRN), and Level Order (BFS).

---

### 5. What is a balanced tree? Why is it important?

A tree where the height difference between left and right subtrees is minimal to ensure O(log n) operations.

---

### 6. What is an AVL tree?

A self-balancing BST maintaining balance factor (-1, 0, 1) with rotations to balance after insert/delete.

---

### 7. What are rotations in AVL trees?

Operations (Left, Right, Left-Right, Right-Left) used to rebalance the tree after insertion or deletion.

---

### 8. What is a Red-Black Tree?

A balanced BST with red/black nodes ensuring balance via color rules, offering O(log n) operations.

---

### 9. How does a Trie work?

A tree structure used for storing strings, where nodes represent characters, allowing fast prefix searches.

---

### 10. What is the height of a binary tree?

The number of edges on the longest path from root to a leaf node.

---

### 11. How do you find the lowest common ancestor (LCA) of two nodes?

By traversing from root, checking if nodes lie in different subtrees, or by storing parent pointers.

---

### 12. What is the difference between DFS and BFS on trees?

DFS explores as far as possible along branches; BFS explores neighbors level by level.

---

### 13. How do you detect if a binary tree is a BST?

Check if inorder traversal yields a sorted sequence or verify node values within valid ranges recursively.

---

### 14. What are leaf nodes?

Nodes with no children.

---

### 15. How do you calculate the number of nodes in a complete binary tree?

Using the formula: number\_of\_nodes = 2^(height + 1) - 1 (if perfect), else traverse and count.

---

### Exercise

Write functions for inorder, preorder, postorder traversals, check if tree is BST, and find LCA of two nodes.

---

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Top 10 CSS Interview Questions

1. What is CSS and what are its key features?
CSS (Cascading Style Sheets) is a stylesheet language used to describe the presentation of a document written in HTML or XML. Its key features include controlling layout, styling text, setting colors, spacing, and more, allowing for a separation of content and design for better maintainability and flexibility.

2. Explain the difference between inline, internal, and external CSS.
- Inline CSS is applied directly within an HTML element using the style attribute.
- Internal CSS is defined within a <style> tag inside the <head> section of an HTML document.
- External CSS is linked to an HTML document via the <link> tag and is written in a separate .css file.

3. What is the CSS box model and what are its components?
The CSS box model describes the rectangular boxes generated for elements in the document tree and consists of four components:
- Content: The actual content of the element.
- Padding: The space between the content and the border.
- Border: The edge surrounding the padding.
- Margin: The space outside the border that separates the element from others.

4. How do you center a block element horizontally using CSS?
To center a block element horizontally, you can use the margin: auto; property. For example:

.center {
  width: 50%;
  margin: auto;
}

5. What are CSS selectors and what are the different types?
CSS selectors are patterns used to select elements to apply styles. The different types include:
- Universal selector (*)
- Element selector (element)
- Class selector (.class)
- ID selector (#id)
- Attribute selector ([attribute])
- Pseudo-class selector (:pseudo-class)
- Pseudo-element selector (::pseudo-element)

6. Explain the difference between absolute, relative, fixed, and sticky positioning in CSS.
- relative: The element is positioned relative to its normal position.
- absolute: The element is positioned relative to its nearest positioned ancestor or the initial containing block if none exists.
- fixed: The element is positioned relative to the viewport and does not move when the page is scrolled.
- sticky: The element is treated as relative until a given offset position is met in the viewport, then it behaves as fixed.

7. What is Flexbox and how is it used in CSS?
Flexbox (Flexible Box Layout) is a layout model that allows for more efficient arrangement of elements within a container. It is used to align and distribute space among items in a container, even when their size is unknown or dynamic. Flexbox is enabled by setting display: flex; on a container element.

8. How do you create a responsive design in CSS?
Responsive design can be achieved using media queries, flexible grid layouts, and relative units like percentages, em, and rem. Media queries adjust styles based on the viewport's width, height, and other characteristics. For example:

@media (max-width: 600px) {
  .container {
    width: 100%;
  }
}

9. What are CSS preprocessors and name a few popular ones.
CSS preprocessors extend CSS with variables, nested rules, and functions, making it more powerful and easier to maintain. Popular CSS preprocessors include:
- Sass (Syntactically Awesome Style Sheets)
- LESS (Leaner Style Sheets)
- Stylus

10. How do you implement CSS animations?
CSS animations are implemented using the @keyframes rule to define the animation and the animation property to apply it to an element. For example:

@keyframes example {
  from {background-color: red;}
  to {background-color: yellow;}
}

.element {
  animation: example 5s infinite;
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1.Components
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• JSX (JavaScript XML) Syntax

2.Props (Properties)
• Passing Props
• Default Props
• Prop Types

3.State
• useState Hook
• Class Component State
• Immutable State

4.Lifecycle Methods (Class Components)
• componentDidMount
• componentDidUpdate
• componentWillUnmount

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• Handling Events in Class Components

7.Conditional Rendering
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8.Lists and Keys
• Rendering Lists
• Keys in React Lists

9.Component Composition
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10.Higher-Order Components (HOC)
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• Using HOCs for Reusability

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In a recently leaked recording, AWS CEO told employees that most developers could stop coding once AI takes over, predicting this is likely to happen within 24 months.

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Important questions to ace your machine learning interview with an approach to answer:

1. Machine Learning Project Lifecycle:
   - Define the problem
   - Gather and preprocess data
   - Choose a model and train it
   - Evaluate model performance
   - Tune and optimize the model
   - Deploy and maintain the model

2. Supervised vs Unsupervised Learning:
   - Supervised Learning: Uses labeled data for training (e.g., predicting house prices from features).
   - Unsupervised Learning: Uses unlabeled data to find patterns or groupings (e.g., clustering customer segments).

3. Evaluation Metrics for Regression:
   - Mean Absolute Error (MAE)
   - Mean Squared Error (MSE)
   - Root Mean Squared Error (RMSE)
   - R-squared (coefficient of determination)

4. Overfitting and Prevention:
   - Overfitting: Model learns the noise instead of the underlying pattern.
   - Prevention: Use simpler models, cross-validation, regularization.

5. Bias-Variance Tradeoff:
   - Balancing error due to bias (underfitting) and variance (overfitting) to find an optimal model complexity.

6. Cross-Validation:
   - Technique to assess model performance by splitting data into multiple subsets for training and validation.

7. Feature Selection Techniques:
   - Filter methods (e.g., correlation analysis)
   - Wrapper methods (e.g., recursive feature elimination)
   - Embedded methods (e.g., Lasso regularization)

8. Assumptions of Linear Regression:
   - Linearity
   - Independence of errors
   - Homoscedasticity (constant variance)
   - No multicollinearity

9. Regularization in Linear Models:
   - Adds a penalty term to the loss function to prevent overfitting by shrinking coefficients.

10. Classification vs Regression:
    - Classification: Predicts a categorical outcome (e.g., class labels).
    - Regression: Predicts a continuous numerical outcome (e.g., house price).

11. Dimensionality Reduction Algorithms:
    - Principal Component Analysis (PCA)
    - t-Distributed Stochastic Neighbor Embedding (t-SNE)

12. Decision Tree:
    - Tree-like model where internal nodes represent features, branches represent decisions, and leaf nodes represent outcomes.

13. Ensemble Methods:
    - Combine predictions from multiple models to improve accuracy (e.g., Random Forest, Gradient Boosting).

14. Handling Missing or Corrupted Data:
    - Imputation (e.g., mean substitution)
    - Removing rows or columns with missing data
    - Using algorithms robust to missing values

15. Kernels in Support Vector Machines (SVM):
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    - Radial Basis Function (RBF) kernel

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