Stack

O(1)O(1)
Code:

class Stack<T> {
    items: T[];
    constructor() {
        this.items = [];
    }

    push(element: T) {
        this.items.push(element);
    }

    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;
    }
}
;