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).
用堆栈实现队列的功能,这里我们借助两个堆栈来模拟队列的先进先出的特性。代码如下:
class MyQueue { Stack<Integer> stack1 = new Stack<Integer>(); Stack<Integer> stack2 = new Stack<Integer>(); // Push element x to the back of queue. 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.isEmpty()) stack1.push(stack2.pop()); } // Get the front element. public int peek() { while(stack1.size() > 1) { stack2.push(stack1.pop()); } int result = stack1.peek(); while(!stack2.isEmpty()) stack1.push(stack2.pop()); return result; } // Return whether the queue is empty. public boolean empty() { return stack1.isEmpty(); } }