[LeetCode]232. Implement Queue using Stacks
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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:很简单,使用两个栈即可,加入新元素时先压到空栈中,然后把另一栈都复制到这个新栈中,再新栈引用给原来栈,就可以了
public class MyQueue { private Stack<Integer> stack; private Stack<Integer> s; /** Initialize your data structure here. */ public MyQueue() { stack=new Stack<Integer>(); s=new Stack<Integer>(); } /** Push element x to the back of queue. */ public void push(int x) { s.push(x); s.addAll(stack); stack=s; s=new Stack<Integer>(); } /** Removes the element from in front of queue and returns that element. */ public int pop() { return stack.pop(); } /** Get the front element. */ public int peek() { return stack.peek(); } /** Returns whether the queue is empty. */ public boolean empty() { return stack.isEmpty(); }}/** * Your MyQueue object will be instantiated and called as such: * MyQueue obj = new MyQueue(); * obj.push(x); * int param_2 = obj.pop(); * int param_3 = obj.peek(); * boolean param_4 = obj.empty(); */
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