Implement a last in first out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal queue (push, top, pop, and empty).
Implement the MyStack class:
- void push(int x) Pushes element x to the top of the stack.
- int pop() Removes the element on the top of the stack and returns it.
- int top() Returns the element on the top of the stack.
- boolean empty() Returns true if the stack is empty, false otherwise.
Notes:
- You must use only standard operations of a queue, which means only push to back, peek/pop from front, size, and is empty operations are valid.
- Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue), as long as you use only a queue’s standard operations.
Example 1:
Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]
Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False
Constraints:
- 1 <= x <= 9
- At most 100 calls will be made to push, pop, top, and empty.
- All the calls to pop and top are valid.
Follow-up: Can you implement the stack 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. You can use more than two queues.
Solution in python:
from collections import deque
class MyStack:
def __init__(self):
"""
Initialize your data structure here.
"""
self.q1 = deque()
self.q2 = deque()
def push(self, x: int) -> None:
"""
Push element x onto stack.
"""
self.q1.append(x)
def pop(self) -> int:
"""
Removes the element on top of the stack and returns that element.
"""
size = len(self.q1)
while size != 1:
temp = self.q1.popleft()
self.q2.append(temp)
size -= 1
temp = self.q1.pop()
self.q1, self.q2 = self.q2, self.q1
return temp
def top(self) -> int:
"""
Get the top element.
"""
size = len(self.q1)
while size != 0:
temp = self.q1.popleft()
self.q2.append(temp)
size -= 1
self.q1, self.q2 = self.q2, self.q1
return temp
def empty(self) -> bool:
"""
Returns whether the stack is empty.
"""
return len(self.q1) == 0
# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()
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