/**
 * Represents a stack data structure.
 * @template T The type of elements in the stack.
 * @example
 * const stack = new Stack<number>();
 * stack.push(1).push(2);
 * console.log(stack.pop()); // 2
 */
export declare class Stack<T> {
    private items;
    private itemSet;
    /**
     * Pushes an item onto the top of the stack.
     * @param {T} item - The item to push onto the stack.
     * @returns {T} The item that was pushed.
     */
    push(item: T): T;
    /**
     * Removes and returns the item at the top of the stack.
     * @returns {T} The item at the top of the stack, or undefined if the stack is empty.
     * @throws {Error} If the stack is empty;
     */
    pop(): T;
    /**
     * Returns the number of items in the stack.
     * @returns {number} The size of the stack.
    */
    size(): number;
    /**
     * Returns the item at the top of the stack without removing it.
     * @returns {T | undefined} The item at the top of the stack, or undefined if the stack is empty.
     */
    peek(): T | undefined;
    select(item: number): T;
    /**
     * Checks if the stack is empty.
     * @returns {boolean} True if the stack is empty, false otherwise.
     */
    isEmpty(): boolean;
    /**
     * Checks if the stack contains a specific item.
     * @param {T} item - The item to check for.
     * @returns {boolean} True if the item is in the stack, false otherwise.
     */
    contains(item: T): boolean;
}