232. Implement Queue using Stacks - Solved in Python/JavaScript/Java/C++/C#/Go/Ruby

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LeetCode link: 232. Implement Queue using Stacks, difficulty: Easy.

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

void push(int x) Pushes element x to the back of the queue.
int pop() Removes the element from the front of the queue and returns it.
int peek() Returns the element at the front of the queue.
boolean empty() Returns true if the queue is empty, false otherwise.

Notes:

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

Example 1:

Input: ["MyQueue", "push", "push", "peek", "pop", "empty"] [[], [1], [2], [], [], []]

Output: [null, null, null, 1, 1, false]

Explanation:

MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: 1, 2
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

Constraints:

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

Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

Intuition
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To implement a queue with two stacks, the intuitive idea is that one stack stack_in is dedicated to push, and the other stack stack_out is dedicated to pop.
Push can be easy, just push directly, then pop is not so easy. How to do it?

Click to view the answer
Stack is last in first out, queue is first in first out, the two are opposite, so it is not possible to pop directly from stack_in. You have to add the elements in stack_in to stack_out in reverse, and then pop.

Complexity

Time complexity

push O(1), pop O(1), peek O(1), empty O(1)

Space complexity

O(n)

Explanation

pop and peek appear to be O(n), but they are actually O(1). This is because if dump_into_stack_out operates on m numbers at a time, then each pop operation for the next m times is O(1).

Python #

class MyQueue:
    def __init__(self):
        self.stack_in = []
        self.stack_out = []

    def push(self, x: int) -> None:
        self.stack_in.append(x)

    def pop(self) -> int:
        self.dump_into_stack_out_if_it_is_empty()
        return self.stack_out.pop()

    def peek(self) -> int:
        self.dump_into_stack_out_if_it_is_empty()
        return self.stack_out[-1]

    def empty(self) -> bool:
        return not self.stack_out and not self.stack_in

    def dump_into_stack_out_if_it_is_empty(self) -> int:
        if not self.stack_out:
            while self.stack_in:
                self.stack_out.append(self.stack_in.pop())

JavaScript #

var MyQueue = function () {
  this.stackIn = []
  this.stackOut = []
};

MyQueue.prototype.push = function (x) {
  this.stackIn.push(x)
};

MyQueue.prototype.pop = function () {
  this.dumpIntoStackOutWhenItIsEmpty()
  return this.stackOut.pop()
};

MyQueue.prototype.peek = function () {
  this.dumpIntoStackOutWhenItIsEmpty()
  return this.stackOut.at(-1)
};

MyQueue.prototype.empty = function () {
  return this.stackOut.length === 0 && this.stackIn.length === 0
};

MyQueue.prototype.dumpIntoStackOutWhenItIsEmpty = function () {
  if (this.stackOut.length === 0) {
    while (this.stackIn.length > 0) {
      this.stackOut.push(this.stackIn.pop())
    }
  }
}

Java #

import java.util.Stack;

class MyQueue {
    private Stack<Integer> stackIn;
    private Stack<Integer> stackOut;

    public MyQueue() {
        stackIn = new Stack<>();
        stackOut = new Stack<>();
    }

    public void push(int x) {
        stackIn.push(x);
    }

    public int pop() {
        dumpIntoStackOutWhenItIsEmpty();
        return stackOut.pop();
    }

    public int peek() {
        dumpIntoStackOutWhenItIsEmpty();
        return stackOut.peek();
    }

    public boolean empty() {
        return stackIn.empty() && stackOut.empty();
    }

    private void dumpIntoStackOutWhenItIsEmpty() {
        if (stackOut.empty()) {
            while (!stackIn.empty()) {
                stackOut.push(stackIn.pop());
            }
        }
    }
}

C++ #

class MyQueue {
private:
    stack<int> stack_in_;
    stack<int> stack_out_;

    void dumpIntoStackOutWhenItIsEmpty() {
        if (stack_out_.empty()) {
            while (!stack_in_.empty()) {
                stack_out_.push(stack_in_.top());
                stack_in_.pop();
            }
        }
    }

public:
    MyQueue() {}

    void push(int x) {
        stack_in_.push(x);
    }

    int pop() {
        dumpIntoStackOutWhenItIsEmpty();
        int value = stack_out_.top();
        stack_out_.pop();
        return value;
    }

    int peek() {
        dumpIntoStackOutWhenItIsEmpty();
        return stack_out_.top();
    }

    bool empty() {
        return stack_in_.empty() && stack_out_.empty();
    }
};

C# #

using System.Collections.Generic;

public class MyQueue
{
    private Stack<int> stackIn;
    private Stack<int> stackOut;

    public MyQueue()
    {
        stackIn = new Stack<int>();
        stackOut = new Stack<int>();
    }

    public void Push(int x)
    {
        stackIn.Push(x);
    }

    public int Pop()
    {
        DumpIntoStackOutWhenItIsEmpty();
        return stackOut.Pop();
    }

    public int Peek()
    {
        DumpIntoStackOutWhenItIsEmpty();
        return stackOut.Peek();
    }

    public bool Empty()
    {
        return stackIn.Count == 0 && stackOut.Count == 0;
    }

    private void DumpIntoStackOutWhenItIsEmpty()
    {
        if (stackOut.Count == 0)
        {
            while (stackIn.Count > 0)
            {
                stackOut.Push(stackIn.Pop());
            }
        }
    }
}

Go #

type MyQueue struct {
  stackIn  []int
  stackOut []int
}

func Constructor() MyQueue {
  return MyQueue{
    stackIn:  make([]int, 0),
    stackOut: make([]int, 0),
  }
}

func (this *MyQueue) Push(x int) {
  this.stackIn = append(this.stackIn, x)
}

func (this *MyQueue) Pop() int {
  this.dumpIntoStackOutWhenItIsEmpty()
  top := this.stackOut[len(this.stackOut) - 1]
  this.stackOut = this.stackOut[:len(this.stackOut) - 1]
  return top
}

func (this *MyQueue) Peek() int {
  this.dumpIntoStackOutWhenItIsEmpty()
  return this.stackOut[len(this.stackOut) - 1]
}

func (this *MyQueue) Empty() bool {
  return len(this.stackIn) == 0 && len(this.stackOut) == 0
}

func (this *MyQueue) dumpIntoStackOutWhenItIsEmpty() {
  if len(this.stackOut) == 0 {
    for len(this.stackIn) > 0 {
      top := this.stackIn[len(this.stackIn) - 1]
      this.stackIn = this.stackIn[:len(this.stackIn) - 1]
      this.stackOut = append(this.stackOut, top)
    }
  }
}

Ruby #

class MyQueue
  def initialize
    @stack_in = []
    @stack_out = []
  end

  def push(x)
    @stack_in.push(x)
  end

  def pop
    dump_into_stack_out_when_it_is_empty
    @stack_out.pop
  end

  def peek
    dump_into_stack_out_when_it_is_empty
    @stack_out.last
  end

  def empty
    @stack_in.empty? && @stack_out.empty?
  end

  private

  def dump_into_stack_out_when_it_is_empty
    if @stack_out.empty?
      while !@stack_in.empty?
        @stack_out.push(@stack_in.pop)
      end
    end
  end
end

Other languages

Welcome to contribute code to LeetCode.blog GitHub -> 232. Implement Queue using Stacks. Thanks!

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