力扣刷题记录 -- JAVA--31--232. 用栈实现队列

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一、题目

二、代码

class MyQueue {

    Stack<Integer> stack_in;
    Stack<Integer> stack_out;
    //核心是: in 和 out 完成反转
    public MyQueue() {

         stack_in = new Stack<>();
         stack_out = new Stack<>();

    }
    
    public void push(int x) {
         stack_in.push(x);
    }
    
    public int pop() 
    {
      //out为空 in 不为空 导入
       empty_stack_in();
       return stack_out.pop();

    }
    
    public int peek() 
    {
        //out为空 in 不为空 导入
        empty_stack_in();
        return stack_out.peek();
    }
    
    public boolean empty() 
    {
         return stack_in.isEmpty() && stack_out.isEmpty();
    }
    public void empty_stack_in()
    {
        if(stack_out.isEmpty()==false) return ;
        while(!stack_in.isEmpty())
        {
            stack_out.push(stack_in.peek());
            stack_in.pop();
        }
    }
}

/**
 * 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();
 */

三、运行结果