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Stack (LIFO)

Stack (LIFO)

Last In, First Out.

Operations

Push ’ O(1)
Pop ’ O(1)
Peek ’ O(1)

Example Flow

push(10) ' [10]
push(20) ' [10, 20]
push(30) ' [10, 20, 30]
pop() ' [10, 20]
peek() ' 20

JavaScript Implementation

class Stack {
  constructor() {
    this.items = [];
  }

  push(value) {
    this.items.push(value);
  }

  pop() {
    if (this.isEmpty()) return null;
    return this.items.pop();
  }

  peek() {
    if (this.isEmpty()) return null;
    return this.items[this.items.length - 1];
  }

  isEmpty() {
    return this.items.length === 0;
  }
}

Real-World Uses

Undo/Redo
Browser history
Function calls
Expression evaluation

Queue (FIFO) — LIFO’s counterpart, used for BFS and task scheduling
Graphs — DFS traversal uses a stack (implicitly via recursion or explicitly)