added linked lists in data structures

Author: Ghruank

Theory/Data-Structures/Linked-Lists/README.md

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+## 3. Linked Lists
+A **Linked List** is a linear data structure consisting of nodes, where each node contains a data part and a reference (or link) to the next node in the sequence.
+
+### Key Characteristics
+- Nodes are stored non-contiguously in memory.
+- Elements are linked using pointers, allowing efficient insertion and deletion.
+
+### Types of Linked Lists
+- **Singly Linked List**: Each node has a single link to the next node.
+- **Doubly Linked List**: Each node has links to both the previous and next nodes.
+- **Circular Linked List**: Last node links back to the first node.
+
+### Common Operations
+- **Insertion**:
+  - At the head, tail, or a specific position.
+  - Complexity: \(O(1)\) for head or tail (if tail pointer exists), \(O(n)\) for arbitrary position.
+- **Deletion**:
+  - Removing an element from head, tail, or specific position.
+  - Complexity: \(O(1)\) for head or tail, \(O(n)\) for arbitrary position.
+- **Traversal**: Accessing elements sequentially.
+  - Complexity: \(O(n)\)
+- **Searching**:
+  - Finding an element.
+  - Complexity: \(O(n)\)
+
+### Applications
+- **Dynamic Memory Management**: Efficient for varying data sizes.
+- **Implementing Stacks and Queues**: Using linked lists as the underlying data structure.
+- **Sparse Matrices**: Efficiently storing and accessing non-zero elements.