232.Implement Queue using Stacks

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Stack, Easy

Question

Implement the following operations of a queue using stacks.

  • push(x) – Push element x to the back of queue.
  • pop() – Removes the element from in front of queue.
  • peek() – Get the front element.
  • empty() – Return whether the queue is empty.

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, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
  • You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

My Answer

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class MyQueue {
    Stack<Integer> queue = new Stack<>();
    /** Initialize your data structure here. */
    public MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        Stack<Integer> temp = new Stack<>();
        while(!queue.isEmpty())
            temp.push(queue.pop());
        queue.push(x);
        while(!temp.isEmpty())
            queue.push(temp.pop());
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return queue.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        return queue.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return queue.isEmpty();
    }
}

Running time: 98ms

More quick one

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class MyQueue {
    Stack<Integer> s1 = new Stack<>();
    Stack<Integer> s2 = new Stack<>();
    /** Initialize your data structure here. */
    public MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (s2.isEmpty()) {
          while (!s1.isEmpty()) {
            s2.push(s1.pop());
          }
        }
        return s2.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        if (s2.isEmpty()) {
          while (!s1.isEmpty()) {
            s2.push(s1.pop());
          }
        }
        return s2.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.isEmpty()&&s2.isEmpty();
    }
}
/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */

Running time: 100ms