Stack

1"""
2Stack - LIFO Data Structure
3
4A stack is a linear data structure that follows the Last In,
5First Out (LIFO) principle. Elements are added and removed
6from the same end, called the "top" of the stack.
7
8Time Complexity:
9  - Push: O(1)
10  - Pop: O(1)
11  - Peek: O(1)
12  - Is Empty: O(1)
13
14Space Complexity: O(n) where n is number of elements
15"""
16
17
18class Stack:
19    def __init__(self):
20        """Initialize an empty stack."""
21        self.items = []
22
23    def push(self, item):
24        """Add an item to the top of the stack."""
25        self.items.append(item)
26
27    def pop(self):
28        """
29        Remove and return the top item from the stack.
30        Raises IndexError if stack is empty.
31        """
32        if self.is_empty():
33            raise IndexError("Cannot pop from empty stack")
34        return self.items.pop()
35
36    def peek(self):
37        """
38        Return the top item without removing it.
39        Raises IndexError if stack is empty.
40        """
41        if self.is_empty():
42            raise IndexError("Cannot peek empty stack")
43        return self.items[-1]
44
45    def is_empty(self):
46        """Return True if the stack is empty."""
47        return len(self.items) == 0
48
49    def size(self):
50        """Return the number of items in the stack."""
51        return len(self.items)
52
53    def __len__(self):
54        """Allow len(stack) syntax."""
55        return self.size()
56
57    def __str__(self):
58        """String representation: bottom -> top."""
59        return f"Stack({self.items})"
60
61
62# Example usage:
63if __name__ == "__main__":
64    stack = Stack()
65
66    # Push elements
67    stack.push(10)
68    stack.push(20)
69    stack.push(30)
70    print(stack)  # Stack([10, 20, 30])
71
72    # Peek at top
73    print(stack.peek())  # 30
74
75    # Pop elements
76    print(stack.pop())  # 30
77    print(stack.pop())  # 20
78    print(stack)  # Stack([10])
79