[LeetCode]232. Implement Queue using Stacks

最新推荐文章于 2021-05-21 15:00:42 发布

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最新推荐文章于 2021-05-21 15:00:42 发布

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分类专栏： LeetCode 文章标签： leetcode

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本文介绍了一种使用两个栈来实现队列的方法。通过将新元素压入一个空栈，并随后将另一个栈的所有元素复制过来，实现了队列的基本操作：push、pop、peek和empty。这种方法确保了操作的有效性和正确性。

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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).

思路：很简单，使用两个栈即可，加入新元素时先压到空栈中，然后把另一栈都复制到这个新栈中，再新栈引用给原来栈，就可以了

public class MyQueue {

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