问题描述:
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.
- 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).
思路:
利用两个栈实现一个队列,模拟队列“先进先出”思想,stack1为入栈,stack2为出栈,入队直接进入stack1即可;出队时,如果stack2有元素,直接是stack2出栈,如果没有元素,则先将stack1的元素放入stack2中,这时最上面的元素出栈即可;获取最顶元素和出队类似;判断队列是否为空时,必须判断两个栈是否都为空才可以。
JAVA代码:
class MyQueue {
// Push element x to the back of queue.
Stack<Integer> stack1 = new Stack<>();
Stack<Integer> stack2 = new Stack<>();
public void push(int x) {
stack1.push(x);
}
// Removes the element from in front of queue.
public void pop() {
if(!stack2.isEmpty()) stack2.pop();
else{
while(!stack1.isEmpty())
stack2.push(stack1.pop());
stack2.pop();
}
}
// Get the front element.
public int peek() {
if(!stack2.isEmpty()) return stack2.peek();
else{
while(!stack1.isEmpty())
stack2.push(stack1.pop());
return stack2.peek();
}
}
// Return whether the queue is empty.
public boolean empty()
{
return stack1.empty() && stack2.empty();
}
}