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  • 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 toppeek/pop from topsize, 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能够了。另外一种思路。进栈的时候不做不论什么处理。出栈的时候把栈逆序放在另外一个栈。出另外一个栈。以下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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  • 原文地址:https://www.cnblogs.com/claireyuancy/p/7159431.html
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