《leetCode》：Implement Stack using Queues

题目

Implement the following operations of a stack using queues.push(x) -- Push element x onto stack.pop() -- Removes the element on top of the stack.top() -- Get the top element.empty() -- Return whether the stack is empty.Notes:You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue),  as long as you use only standard operations of a queue.You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

思路

此题比较简单，借用两个队列来实现栈即可，要明白队列是：先进先出，而栈是：先进后出。明白了这两点就比较好实现了。

class MyStack {    Queue<Integer> q1=new LinkedList<Integer>();    Queue<Integer> q2=new LinkedList<Integer>();    // Push element x onto stack.    public void push(int x) {        q1.add(x);    }    // Removes the element on top of the stack.    public void pop() {        if(q1.isEmpty()){            return;        }        else{            int len=q1.size();            for(int i=0;i<len-1;i++){                q2.add(q1.poll());            }            q1.poll();//最后一个元素不要。            for(int i=0;i<len-1;i++){                q1.add(q2.poll());            }        }    }    // Get the top element.    public int top() {        if(q1.isEmpty()){            return -1;        }        else{            int len=q1.size();            for(int i=0;i<len-1;i++){                q2.add(q1.poll());            }            int res=q1.poll();//保存最后一个元素。            q2.add(res);            for(int i=0;i<len;i++){                q1.add(q2.poll());            }            return res;        }    }    // Return whether the stack is empty.    public boolean empty() {        return q1.isEmpty();    }}

小结

在回寝室前，在leetcode上AC了这两道简单的题，还是挺愉快的哈。