Leetcode232. Implement Queue using Stacks[栈实现队列]

一、题目描述

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)

二、考察点

栈和队列

三、CODE描述

栈和队列

import java.util.Stack;
class MyQueue {

    // Push element x to the back of queue.
    Stack<Integer> stack1 =new Stack<Integer>(); // push
    Stack<Integer> stack2 =new Stack<Integer>(); // pop
    public void push(int x) {
    	stack1.push(x);      
    }

    // Removes the element from in front of queue.
    public void pop() {
    	if(stack2.empty()){
    		while(!stack1.empty()){
    			int top = stack1.peek();
    			stack1.pop();
    			stack2.push(top);
    		}
    	}
    	if(!stack2.empty()){  
        	stack2.pop();   
    	}
    }

    // Get the front element.
    public int peek() {
    	if(stack2.empty()){
    		while(!stack1.empty()){
    			int top = stack1.peek();
    			stack1.pop();
    			stack2.push(top);
    		}
    	}
    	if(stack2.empty()) return -1;
        else    	
    	return stack2.peek(); 
    	
    }

    // Return whether the queue is empty.
    public boolean empty() {
        
    	return stack1.empty()&&stack2.empty();
    }
}

四、结果

越努力，越幸运。