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
, andis 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).
代码如下:
class MyQueue { // Push element x to the back of queue. Stack<Integer> stack1=new Stack<Integer>(); Stack<Integer> stack2=new Stack<Integer>(); public void push(int x) { stack1.push(x); } // Removes the element from in front of queue. public void pop() { while(stack1.size()>1) stack2.push(stack1.pop()); stack1.pop(); while(stack2.size()>0) stack1.push(stack2.pop()); } // Get the front element. public int peek() { while(stack1.size()>1) stack2.push(stack1.pop()); int x= stack1.peek(); while(stack2.size()>0) stack1.push(stack2.pop()); return x; } // Return whether the queue is empty. public boolean empty() { return stack1.empty(); } }
运行结果: