232. Implement Queue using Stacks

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Total Submissions: 97820
Difficulty: Easy

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).

两个栈来实现队列的效果

Java:

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class MyQueue {
    Stack<Integer> input = new Stack();
    Stack<Integer> output = new Stack();

    // Push element x to the back of queue.
    public void push(int x) {
        input.push(x);
    }

    // Removes the element from in front of queue.
    public void pop() {
        peek();
        output.pop();
    }

    // Get the front element.
    public int peek() {
        if (output.empty())
            while (!input.empty())
                output.push(input.pop());
        return output.peek();
    }

    // Return whether the queue is empty.
    public boolean empty() {
         return input.empty() && output.empty();
    }
}

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