232. Implement Queue using Stacks LeetCode Solution

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Table of Contents

1. Problem Statement
2. Complexity Analysis
3. Implement Queue using Stacks solution in C++
4. Implement Queue using Stacks solution in Java
5. Implement Queue using Stacks solution in Python
6. Additional Resources

Problem Statement of Implement Queue using Stacks

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] (leftmost is front of the queue)
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.

Complexity Analysis

• Time Complexity: O(1)
• Space Complexity: O(n)

232. Implement Queue using Stacks LeetCode Solution in C++

class MyQueue {
 public:
  void push(int x) {
    input.push(x);
  }

  int pop() {
    peek();
    const int val = output.top();
    output.pop();
    return val;
  }

  int peek() {
    if (output.empty())
      while (!input.empty())
        output.push(input.top()), input.pop();
    return output.top();
  }

  bool empty() {
    return input.empty() && output.empty();
  }

 private:
  stack<int> input;
  stack<int> output;
};
/* code provided by PROGIEZ */

232. Implement Queue using Stacks LeetCode Solution in Java

class MyQueue {
  public void push(int x) {
    input.push(x);
  }

  public int pop() {
    peek();
    return output.pop();
  }

  public int peek() {
    if (output.isEmpty())
      while (!input.isEmpty())
        output.push(input.pop());
    return output.peek();
  }

  public boolean empty() {
    return input.isEmpty() && output.isEmpty();
  }

  private Deque<Integer> input = new ArrayDeque<>();
  private Deque<Integer> output = new ArrayDeque<>();
}
// code provided by PROGIEZ

232. Implement Queue using Stacks LeetCode Solution in Python

class MyQueue:
  def __init__(self):
    self.input = []
    self.output = []

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

  def pop(self) -> int:
    self.peek()
    return self.output.pop()

  def peek(self) -> int:
    if not self.output:
      while self.input:
        self.output.append(self.input.pop())
    return self.output[-1]

  def empty(self) -> bool:
    return not self.input and not self.output
 # code by PROGIEZ

Additional Resources

• Explore all LeetCode problem solutions at Progiez here
• Explore all problems on LeetCode website here

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