ACAlgoGeeks

Algorithm Visualizer

▤Array

Two Pointers 3Sum Trapping Rain Water Sliding Window Max Longest Substring Without Repeating Characters Binary Search

▤Grid traversal

Breadth-First Search (BFS)Depth-First Search (DFS)

◈Graph

Dijkstra's Shortest Path

◎Pathfinding

A* Pathfinding

⬡Data Structure

Segment Tree LRU Cache Union-Find Min Heap Stack

⊕Hash-Based

Bloom Filter Consistent Hashing

♟Game Theory

Minimax + α-β

●Dynamic Programming

Maximum Subarray (Kadane's)0/1 Knapsack

●Greedy

Activity Selection

Stack

Data Structure

T: O(1) per push/popS: O(n)

Feedback

Stack

Stack initialized (empty).

Stack (top → bottom)

empty

PUSH(10)

PUSH(20)

PUSH(5)

POP()

PUSH(15)

POP()

SpeedNormal (500ms)

Parameters

{ operations: [{ type: "push", value: number } | { type: "pop" }] }

Variables

stack[]

opIndex0

currentOppush(10)

1push(x):
2  stack.append(x)
3 
4pop():
5  if stack empty: error
6  return stack.removeLast()

1const stack: number[] = [];
2function push(x: number) { stack.push(x); }
3function pop(): number {
4  if (stack.length === 0) throw new Error('empty');
5  return stack.pop()!;
6}

About

Stack is a LIFO (Last In, First Out) data structure. Push adds an element on top; pop removes and returns the top element. Used in expression parsing, DFS, and undo buffers.

Step-by-step explanation

Step 1 — Invariant: A stack maintains a single "top" of the stack. The last element pushed is the first one to be popped (LIFO). We can implement it with an array (or linked list) by adding and removing only from one end.

Step 2 — Push: To push a value, add it to the top of the stack (e.g. append to the end of the array and increment the top index, or create a new node and set it as the new head in a linked list). The new element becomes the top. Time O(1).

Step 3 — Pop: To pop, remove and return the top element. If the stack is empty, pop is undefined or throws. After removal, the previous element (if any) becomes the new top. Time O(1).

Step 4 — Applications: Stacks are used for depth-first search (DFS), matching brackets in expressions, evaluating postfix expressions, and maintaining undo history. Peek (or top) returns the top without removing it; isEmpty checks if the stack has no elements.

What's happening

Stack initialized (empty).

Time Complexity

O(1) per push/pop

Space Complexity

O(n)

Output

Stack initialized (empty).