232. Implement Queue using Stacks

• Time:
  • Constructor: $O(1)$
  • push(x: int): $O(1)$
  • pop(): $O(n)$
  • peek(): $O(n)$
  • empty(): $O(1)$
• Space: $O(n)$

C++JavaPython

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;
};

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<>();
}

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