Add helpers module with data structures, algorithms, and interactive demos

helpers/README.md

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+# LeetCode Helper Functions
+
+This folder contains utility functions and data structures to help with debugging and solving LeetCode problems.
+
+## 📚 Contents
+
+- **[data_structures.py](data_structures.py)**: Common data structures (TreeNode, ListNode)
+- **[tree_utils.py](tree_utils.py)**: Binary tree helper functions
+- **[linked_list_utils.py](linked_list_utils.py)**: Linked list helper functions
+- **[algorithms.py](algorithms.py)**: Common algorithms (BFS, DFS, tree search)
+
+## 🚀 Demo Notebooks
+
+Interactive Jupyter notebooks demonstrating the helper functions:
+
+- **[Binary Trees Demo](demo_binary_trees.ipynb)** [![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/ContextLab/leetcode-solutions/blob/main/helpers/demo_binary_trees.ipynb)
+- **[Linked Lists Demo](demo_linked_lists.ipynb)** [![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/ContextLab/leetcode-solutions/blob/main/helpers/demo_linked_lists.ipynb)
+
+## 📖 Quick Start
+
+### Installation
+
+From the root of the repository:
+
+```python
+from helpers import *
+```
+
+### Binary Trees
+
+#### Creating Trees
+
+```python
+from helpers import list_to_tree, print_tree
+
+# Create a tree from a list (LeetCode format)
+tree = list_to_tree([1, 2, 3, 4, 5, None, 7])
+
+# Visualize it
+print_tree(tree)
+# Output:
+# Root: 1
+# ├─ L: 2
+# │  ├─ L: 4
+# │  └─ R: 5
+# └─ R: 3
+#    └─ R: 7
+```
+
+#### Converting Trees
+
+```python
+from helpers import tree_to_list
+
+# Convert a tree back to a list
+tree_list = tree_to_list(tree)
+print(tree_list)  # [1, 2, 3, 4, 5, None, 7]
+```
+
+#### Tree Traversals
+
+```python
+from helpers import bfs_traversal, dfs_preorder, dfs_inorder, dfs_postorder
+
+tree = list_to_tree([1, 2, 3, 4, 5])
+
+print(bfs_traversal(tree))    # [1, 2, 3, 4, 5]
+print(dfs_preorder(tree))     # [1, 2, 4, 5, 3]
+print(dfs_inorder(tree))      # [4, 2, 5, 1, 3]
+print(dfs_postorder(tree))    # [4, 5, 2, 3, 1]
+```
+
+### Linked Lists
+
+#### Creating Linked Lists
+
+```python
+from helpers import list_to_linked_list, print_linked_list
+
+# Create a linked list from a Python list
+head = list_to_linked_list([1, 2, 3, 4, 5])
+
+# Visualize it
+print_linked_list(head)
+# Output: 1 -> 2 -> 3 -> 4 -> 5 -> None
+```
+
+#### Converting Linked Lists
+
+```python
+from helpers import linked_list_to_list
+
+# Convert back to a Python list
+result = linked_list_to_list(head)
+print(result)  # [1, 2, 3, 4, 5]
+```
+
+#### Accessing Nodes
+
+```python
+from helpers import get_node_at_index, get_linked_list_length
+
+head = list_to_linked_list([10, 20, 30, 40, 50])
+
+# Get length
+length = get_linked_list_length(head)  # 5
+
+# Get node at index
+node = get_node_at_index(head, 2)
+print(node.val)  # 30
+```
+
+## 📋 Complete Function Reference
+
+### Data Structures
+
+#### TreeNode
+```python
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None)
+```
+Standard binary tree node used in LeetCode problems.
+
+#### ListNode
+```python
+class ListNode:
+    def __init__(self, val=0, next=None)
+```
+Standard linked list node used in LeetCode problems.
+
+### Binary Tree Functions
+
+#### Conversion Functions
+- `list_to_tree(values)`: Convert a list to a binary tree (level-order)
+- `tree_to_list(root)`: Convert a binary tree to a list (level-order)
+
+#### Visualization Functions
+- `print_tree(root)`: Print a visual representation of a tree
+- `visualize_tree(root)`: Get a string representation of a tree
+
+#### Tree Properties
+- `get_tree_height(root)`: Get the height of a tree
+- `count_nodes(root)`: Count total nodes in a tree
+
+### Linked List Functions
+
+#### Conversion Functions
+- `list_to_linked_list(values)`: Convert a Python list to a linked list
+- `linked_list_to_list(head)`: Convert a linked list to a Python list
+
+#### Visualization Functions
+- `print_linked_list(head)`: Print a visual representation of a linked list
+
+#### List Properties
+- `get_linked_list_length(head)`: Get the length of a linked list
+- `get_node_at_index(head, index)`: Get the node at a specific index
+
+### Algorithm Functions
+
+#### Tree Traversals
+- `bfs_traversal(root)`: Breadth-first search (level-order) traversal
+- `dfs_preorder(root)`: Depth-first search pre-order traversal
+- `dfs_inorder(root)`: Depth-first search in-order traversal
+- `dfs_postorder(root)`: Depth-first search post-order traversal
+- `level_order_traversal(root)`: Level-order traversal grouped by level
+
+#### Tree Search
+- `find_path_to_node(root, target)`: Find path from root to a target node
+- `lowest_common_ancestor(root, p, q)`: Find the lowest common ancestor of two nodes
+- `search_bst(root, target)`: Search for a value in a Binary Search Tree
+- `is_valid_bst(root)`: Check if a tree is a valid Binary Search Tree
+
+## 💡 Usage Examples
+
+### Example 1: Testing Your Solution
+
+```python
+from helpers import list_to_tree, tree_to_list
+
+def your_solution(root):
+    # Your solution code here
+    pass
+
+# Test with LeetCode test case
+test_input = [1, 2, 3, 4, 5, None, 7]
+tree = list_to_tree(test_input)
+result = your_solution(tree)
+print(tree_to_list(result))
+```
+
+### Example 2: Debugging Tree Structure
+
+```python
+from helpers import list_to_tree, print_tree, get_tree_height, count_nodes
+
+tree = list_to_tree([1, 2, 3, 4, 5, 6, 7, 8])
+
+print("Tree structure:")
+print_tree(tree)
+
+print(f"\nHeight: {get_tree_height(tree)}")
+print(f"Total nodes: {count_nodes(tree)}")
+```
+
+### Example 3: Visualizing Linked List Operations
+
+```python
+from helpers import list_to_linked_list, print_linked_list
+
+# Before operation
+head = list_to_linked_list([1, 2, 3, 4, 5])
+print("Before:")
+print_linked_list(head)
+
+# Your operation here (e.g., reverse)
+# ...
+
+print("\nAfter:")
+print_linked_list(head)
+```
+
+### Example 4: Understanding Tree Traversals
+
+```python
+from helpers import list_to_tree, print_tree
+from helpers import bfs_traversal, dfs_preorder, dfs_inorder, dfs_postorder
+
+tree = list_to_tree([1, 2, 3, 4, 5, 6, 7])
+
+print("Tree:")
+print_tree(tree)
+print()
+
+print("Traversals:")
+print(f"BFS:        {bfs_traversal(tree)}")
+print(f"Pre-order:  {dfs_preorder(tree)}")
+print(f"In-order:   {dfs_inorder(tree)}")
+print(f"Post-order: {dfs_postorder(tree)}")
+```
+
+## 🤝 Contributing
+
+Feel free to add more helper functions as you encounter common patterns in LeetCode problems!
+
+To add a new helper:
+1. Add the function to the appropriate module (`tree_utils.py`, `linked_list_utils.py`, etc.)
+2. Update `__init__.py` to export the function
+3. Add documentation and examples
+4. Update this README
+
+## 📝 Notes
+
+- All functions follow LeetCode's standard conventions for data structures
+- `None` in tree lists represents null nodes (LeetCode format)
+- Functions are designed to be copy-paste friendly for LeetCode submissions
+- Visualization functions are great for debugging but won't work on LeetCode (they don't affect solutions)
+
+## 🔗 Related Resources
+
+- [LeetCode Official Site](https://leetcode.com)
+- [Main Repository README](../README.md)
+- [Problems We've Solved](../problems)
+
+Happy coding! 🎉

helpers/__init__.py

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+"""
+LeetCode Helper Functions
+
+This package provides utility functions and data structures for solving LeetCode problems.
+
+Main modules:
+- data_structures: Common data structures (TreeNode, ListNode)
+- tree_utils: Binary tree utilities (conversions, visualization)
+- linked_list_utils: Linked list utilities (conversions, helpers)
+- algorithms: Common algorithms (BFS, DFS, tree search)
+"""
+
+from .data_structures import TreeNode, ListNode
+from .tree_utils import (
+    list_to_tree,
+    tree_to_list,
+    print_tree,
+    visualize_tree,
+    get_tree_height,
+    count_nodes
+)
+from .linked_list_utils import (
+    list_to_linked_list,
+    linked_list_to_list,
+    print_linked_list,
+    get_linked_list_length,
+    get_node_at_index
+)
+from .algorithms import (
+    bfs_traversal,
+    dfs_preorder,
+    dfs_inorder,
+    dfs_postorder,
+    level_order_traversal,
+    find_path_to_node,
+    lowest_common_ancestor,
+    search_bst,
+    is_valid_bst
+)
+
+__all__ = [
+    # Data structures
+    'TreeNode',
+    'ListNode',
+    # Tree utilities
+    'list_to_tree',
+    'tree_to_list',
+    'print_tree',
+    'visualize_tree',
+    'get_tree_height',
+    'count_nodes',
+    # Linked list utilities
+    'list_to_linked_list',
+    'linked_list_to_list',
+    'print_linked_list',
+    'get_linked_list_length',
+    'get_node_at_index',
+    # Algorithms
+    'bfs_traversal',
+    'dfs_preorder',
+    'dfs_inorder',
+    'dfs_postorder',
+    'level_order_traversal',
+    'find_path_to_node',
+    'lowest_common_ancestor',
+    'search_bst',
+    'is_valid_bst',
+]