232. Implement Queue using Stacks

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).

三个堆辅助

class MyQueue {
    Stack<Integer> s1 = new Stack<Integer>();
    Stack<Integer> s2 = new Stack<Integer>();
    Stack<Integer> s3 = new Stack<Integer>();
    /** Initialize your data structure here. */
    public MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        if (s1.isEmpty()) {
            s1.push(x);
            while (!s2.isEmpty()) s3.push(s2.pop());
            while (!s3.isEmpty()) s1.push(s3.pop());
        } else {
            s2.push(x);
            while (!s1.isEmpty()) s3.push(s1.pop());
            while (!s3.isEmpty()) s2.push(s3.pop());
        }
        
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (!s1.isEmpty()) return s1.pop();
        else return s2.pop();
        
       }
    
    /** Get the front element. */
    public int peek() {
        if (!s1.isEmpty()) return s1.peek();
        else 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();
 */

使用第三个堆作为暂时的存储，保证前两个堆其中之一的top是第一个元素。

两个堆辅助

class MyQueue {
    Stack<Integer> s1 = new Stack<Integer>();
    Stack<Integer> s2 = new Stack<Integer>();
        /** 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();
 */

解题思路

• 使用两个堆辅助，s2的元素顺序是队列的顺序（先进先出）；
• 新的元素都push到s1，在pop和peek时检查s2是否为空，若为空则将s1中的元素pop出来再push如s2；否则直接pop或者peeks2的堆顶元素。