LeetCode232 Implement Queue using Stacks Java 题解

题目：

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 emptyoperations 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).

解题：

用栈实现队列，这个比用队列实现栈要麻烦一些，这里用到两个栈，两种思路，第一种就是在进栈的时候，把栈逆序放在另外一个栈，出的时候直接出另外一个栈就可以了。第二种思路，进栈的时候不做任何处理，出栈的时候把栈逆序放在另外一个栈，出另外一个栈。下面就是两种的代码实现。

在进栈的时候进行处理：

class MyQueue {
    // Push element x to the back of queue.
	Stack<Integer> stack=new Stack<>();
	Stack<Integer> stack2=new Stack<>();

    public void push(int x) {
    	while(!stack.isEmpty())
    	{
    		stack2.push(stack.pop());
    	}
    	stack2.push(x);
    	while(!stack2.isEmpty())
    	{
    		stack.push(stack2.pop());
    	}

    }

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

    // Get the front element.
    public int peek() {
        return stack.peek();
    }

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

在出栈的时候进行处理：

class MyQueue2 {
    // Push element x to the back of queue.
	Stack<Integer> stack=new Stack<>();
	Stack<Integer> stack2=new Stack<>();

    public void push(int x) {
    	while(!stack2.isEmpty())
    		stack.push(stack2.pop());
    	stack.push(x);

    }

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

    	while(!stack.isEmpty())
    		stack2.push(stack.pop());
        stack2.pop();

    }

    // Get the front element.
    public int peek() {
    	while(!stack.isEmpty())
    		stack2.push(stack.pop());
        return stack2.peek();

    }

    // Return whether the queue is empty.
    public boolean empty() {
    	while(!stack2.isEmpty())
    		stack.push(stack2.pop());
        return stack.isEmpty();
    }
}

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