LeetCode Notes: Implement Stack using Queues

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Question

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

  • void push(int x) Pushes element x to the top of the stack.
  • int pop() Removes the element on the top of the stack and returns it.
  • int top() Returns the element on the top of the stack.
  • boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

  • You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
  • Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.

Example 1:

Input

["MyStack", "push", "push", "top", "pop", "empty"]

[[], [1], [2], [], [], []]

Output

[null, null, null, 2, 2, false]

Explanation

MyStack myStack = new MyStack();

myStack.push(1);

myStack.push(2);

myStack.top(); // return 2

myStack.pop(); // return 2

myStack.empty(); // return False

Constraints:

  • 1 <= x <= 9
  • At most 100 calls will be made to push, pop, top, and empty.
  • All the calls to pop and top are valid.

Follow-up: Can you implement the stack using only one queue?

For questions of the same type, please refer to

232. Use stacks to implement queues

Solution

Analysis:

Take advantage of the characteristics of the js array, where the top of the stack is at the end of the array.

Code:

/**
 * Initialize your data structure here.
 */
var MyStack = function() {
    this.stack = [];
};

/**
 * Push element x onto stack. 
 * @param {number} x
 * @return {void}
 */
MyStack.prototype.push = function(x) {
    this.stack.push(x)
};

/**
 * Removes the element on top of the stack and returns that element.
 * @return {number}
 */
MyStack.prototype.pop = function() {
    return this.stack.pop()
};

/**
 * Get the top element.
 * @return {number}
 */
MyStack.prototype.top = function() {
    return this.stack[this.stack.length - 1]
};

/**
 * Returns whether the stack is empty.
 * @return {boolean}
 */
MyStack.prototype.empty = function() {
    return this.stack.length === 0
};

/**
 * Your MyStack object will be instantiated and called as such:
 * var obj = new MyStack()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.top()
 * var param_4 = obj.empty()
 */

Reference

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