Leetcode 232 Implement Queue using Stacks

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).

这个还是挺有意思的 值得注意的是可以使用多个stack 并且要注意空栈的特殊情况

另一个用queue表示stack 

Leetcode 225 Implement Stack using Queues

class MyQueue {    // Push element x to the back of queue.    Stack<Integer> in  = new Stack();    Stack<Integer> out  = new Stack();    public void push(int x) {        in.push(x);    }    // Removes the element from in front of queue.    public void pop() {        peek();//pop的操作 需要考虑output是否为空栈        out.pop();    }    // Get the front element.    public int peek() {        if(out.empty()){            while(!in.empty()){                out.push(in.pop());            }        }        return out.peek();    }    // Return whether the queue is empty.    public boolean empty() {        return in.empty() && out.empty();    }}