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leetcode 225. implement stack using queues

admin 11月 12, 2020 0

算法
Java
Leetcode

Question

Implement the following operations of a stack using queues.

  • push(x) —— Push element x onto stack.
  • pop() —— Remove the element on top of the stack.
  • top() —— Get the top element.
  • empty() —— Return whether the stack is empty.

See it on Leetcode

Notes

  1. You must use only standard operations of a queue —— which means only push to back, peek/pop from front, size and is empty operations are valid.
  2. Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque(double-ended que), as long as you use only standard operations of a queue.
  3. You may assume that all operations are valid(eg. no pop or top operations will be called on an empty stack).

Solution

  • java
  • cpp
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class  {  
    private Queue<Integer> q1 = new LinkedList<Integer>();
    private Queue<Integer> q2 = new LinkedList<Integer>();
    
    
    public void push(int x) {
        if (q1.isEmpty()) {
            q1.offer(x);
            while (!q2.isEmpty()) q1.offer(q2.poll());
        } else {
            q2.offer(x);
            while (!q1.isEmpty()) q2.offer(q1.poll());
        }
    }

    // Remove the element on top of the stack.
    public void pop() {
        if (!q1.isEmpty()) q1.poll();
        if (!q2.isEmpty()) q2.poll();
    }

    // Get the top element.
    public int top() {
        if (!q1.isEmpty()) {
            return q1.peek();
        } else {
            return q2.peek();
        }
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return q1.isEmpty() && q2.isEmpty();
    } 
}

Editorial Solution

  • java
  • cpp
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class  {  
    private LinkedList<Integer> q1 = new LinkedList<>();

    
    public void push(int x) {
        q1.add(x);
        int size = q1.size();
        while (size > 1) {
            q1.add(q1.remove());
            size--;
        }
    }

    // Remove the element on top of the stack.
    public void pop() {
        q1.remove();
    }

    // Get the top element.
    public int top() {
        return q1.peek();
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return q1.isEmpty();
    }
}
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