Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (
void push(int x)Pushes element x to the back of the queue.
int pop()Removes the element from the front of the queue and returns it.
int peek()Returns the element at the front of the queue.
trueif the queue is empty,
- You must use only standard operations of a stack, which means only
push to top,
peek/pop from top,
is emptyoperations are valid.
- Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack’s standard operations.
Follow-up: Can you implement the queue such that each operation is amortized
O(1) time complexity? In other words, performing
n operations will take overall
O(n) time even if one of those operations may take longer.
Input ["MyQueue", "push", "push", "peek", "pop", "empty"] [, , , , , ] Output
[null, null, null, 1, 1, false]
Explanation MyQueue myQueue = new MyQueue(); myQueue.push(1); // queue is:  myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue) myQueue.peek(); // return 1 myQueue.pop(); // return 1, queue is  myQueue.empty(); // return false
1 <= x <= 9
- At most
100calls will be made to
- All the calls to
Using two stacks, one stack is used for appending items, the other stack is used for pop() and peek().
class MyQueue: def __init__(self): """ Initialize your data structure here. """ self.stack1 =  self.stack2 =  def push(self, x: int) -> None: """ Push element x to the back of queue. """ self.stack1.append(x) def pop(self) -> int: """ Removes the element from in front of queue and returns that element. """ if self.stack2: return self.stack2.pop() else: while self.stack1: self.stack2.append(self.stack1.pop()) return self.stack2.pop() def peek(self) -> int: """ Get the front element. """ if self.stack2: return self.stack2[-1] else: while self.stack1: self.stack2.append(self.stack1.pop()) return self.stack2[-1] def empty(self) -> bool: """ Returns whether the queue is empty. """ return not self.stack1 and not self.stack2 # 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()
- Time complexity: O(N).
- Space complexity: O(1).