added stacks and queues in data structures

Author: Ghruank

Theory/Data-Structures/Queues/README.md

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+## 2. Queues
+A **Queue** is a linear data structure that follows the First-In-First-Out (FIFO) principle. The first element added is the first to be removed.
+
+### Key Characteristics
+- Operates in a **FIFO** order.
+- Can be implemented using arrays, linked lists, or circular buffers.
+
+### Types of Queues
+- **Simple Queue**: Basic FIFO queue.
+- **Circular Queue**: The last position is connected back to the first.
+- **Priority Queue**: Elements are dequeued based on priority.
+- **Deque**: Double-ended queue, allowing insertion and deletion at both ends.
+
+### Common Operations
+- **Enqueue(x)**: Adds element `x` at the end of the queue.
+  - Complexity: \(O(1)\)
+- **Dequeue()**: Removes and returns the front element.
+  - Complexity: \(O(1)\)
+- **Front()**: Returns the front element without removing it.
+  - Complexity: \(O(1)\)
+- **isEmpty()**: Checks if the queue is empty.
+  - Complexity: \(O(1)\)
+
+### Applications
+- **CPU Scheduling**: Round-robin scheduling.
+- **Breadth-First Search (BFS)**: Used in graph traversal.
+- **IO Buffers**: Handling asynchronous data.

Theory/Data-Structures/Stacks/README.md

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+## 1. Stacks
+A **Stack** is a linear data structure that follows the Last-In-First-Out (LIFO) principle. This means the last element added is the first one to be removed.
+
+### Key Characteristics
+- Operates in a **LIFO** order.
+- Can be implemented using arrays or linked lists.
+
+### Common Operations
+- **Push(x)**: Adds element `x` to the top of the stack.
+  - Complexity: \(O(1)\)
+- **Pop()**: Removes and returns the top element.
+  - Complexity: \(O(1)\)
+- **Peek/Top()**: Returns the top element without removing it.
+  - Complexity: \(O(1)\)
+- **isEmpty()**: Checks if the stack is empty.
+  - Complexity: \(O(1)\)
+
+### Applications
+- **Expression Parsing**: Evaluating postfix and infix expressions.
+- **Function Calls**: Recursive calls use a call stack.
+- **Backtracking**: Helps in scenarios where previous states are revisited, like maze solving or browser history.