LeetCode 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.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

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).

使用栈实现队列功能。

考虑辅助栈。

//java
class MyQueue {
    Stack<Integer> pushStack = new Stack<> ();
    Stack<Integer> popStack = new Stack<> ();

    /** Initialize your data structure here. */
    public MyQueue() {

    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        while (!popStack.isEmpty()) {
            pushStack.push(popStack.pop());
        }
        pushStack.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        while (!pushStack.isEmpty()) {
            popStack.push(pushStack.pop());
        }
        return popStack.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        while (!pushStack.isEmpty()) {
            popStack.push(pushStack.pop());
        }
        return popStack.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return pushStack.isEmpty() && popStack.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();
 */