请你仅使用两个栈实现先入先出队列。队列应当支持一般队列支持的所有操作(push
、pop
、peek
、empty
):
实现 MyQueue
类:
void push(int x)
将元素 x 推到队列的末尾int pop()
从队列的开头移除并返回元素int peek()
返回队列开头的元素boolean empty()
如果队列为空,返回true
;否则,返回false
说明:
- 你只能使用标准的栈操作 —— 也就是只有
push to top
,peek/pop from top
,size
, 和is empty
操作是合法的。 - 你所使用的语言也许不支持栈。你可以使用 list 或者 deque(双端队列)来模拟一个栈,只要是标准的栈操作即可。
进阶:
- 你能否实现每个操作均摊时间复杂度为
O(1)
的队列?换句话说,执行n
个操作的总时间复杂度为O(n)
,即使其中一个操作可能花费较长时间。
示例:
输入: ["MyQueue", "push", "push", "peek", "pop", "empty"] [[], [1], [2], [], [], []] 输出: [null, null, null, 1, 1, false] 解释: MyQueue myQueue = new MyQueue(); myQueue.push(1); // queue is: [1] myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue) myQueue.peek(); // return 1 myQueue.pop(); // return 1, queue is [2] myQueue.empty(); // return false
提示:
1 <= x <= 9
- 最多调用
100
次push
、pop
、peek
和empty
- 假设所有操作都是有效的 (例如,一个空的队列不会调用
pop
或者peek
操作)
class MyQueue:
def __init__(self):
"""
Initialize your data structure here.
"""
self.s1 = []
self.s2 = []
def push(self, x: int) -> None:
"""
Push element x to the back of queue.
"""
self.s1.append(x)
def pop(self) -> int:
"""
Removes the element from in front of queue and returns that element.
"""
self._move()
return self.s2.pop()
def peek(self) -> int:
"""
Get the front element.
"""
self._move()
return self.s2[-1]
def empty(self) -> bool:
"""
Returns whether the queue is empty.
"""
return len(self.s1) + len(self.s2) == 0
def _move(self):
"""
Move elements from s1 to s2.
"""
if len(self.s2) == 0:
while len(self.s1) > 0:
self.s2.append(self.s1.pop())
# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()
class MyQueue {
private Deque<Integer> s1 = new ArrayDeque<>();
private Deque<Integer> s2 = new ArrayDeque<>();
/** Initialize your data structure here. */
public MyQueue() {
}
/** Push element x to the back of queue. */
public void push(int x) {
s1.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
move();
return s2.pop();
}
/** Get the front element. */
public int peek() {
move();
return s2.peek();
}
/** Returns whether the queue is empty. */
public boolean empty() {
return s1.isEmpty() && s2.isEmpty();
}
/** Move elements from s1 to s2. */
private void move() {
if (s2.isEmpty()) {
while (!s1.isEmpty()) {
s2.push(s1.pop());
}
}
}
}
/**
* 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();
*/
class MyQueue {
stack1: number[];
stack2: number[];
constructor() {
this.stack1 = [];
this.stack2 = [];
}
push(x: number): void {
this.stack1.push(x);
}
pop(): number {
if (!this.stack2.length) {
while (this.stack1.length) {
this.stack2.push(this.stack1.pop());
}
}
return this.stack2.pop();
}
peek(): number {
if (!this.stack2.length) {
while (this.stack1.length) {
this.stack2.push(this.stack1.pop());
}
}
return this.stack2[this.stack2.length - 1];
}
empty(): boolean {
return !this.stack1.length && !this.stack2.length;
}
}
/**
* Your MyQueue object will be instantiated and called as such:
* var obj = new MyQueue()
* obj.push(x)
* var param_2 = obj.pop()
* var param_3 = obj.peek()
* var param_4 = obj.empty()
*/