#225

Implement Stack using Queues

pupil · 305 · lc easy +21 · 69.4% accepted · 6,897 likes · top 77%

Description

Build a LIFO (last-in-first-out) stack using only two queues as the underlying data structure. The stack must support four operations: push, pop, top, and empty.

Implement the MyStack class:

- void push(int x) Adds element x to the top of the stack.

- int pop() Removes and returns the top element.

- int top() Returns the top element without removing it.

- boolean empty() Returns true when the stack is empty, false otherwise.

Constraints:

- Only standard queue operations are permitted: enqueue to back, peek/dequeue from front, size check, and empty check.

- If your language lacks a native queue, a list or deque used strictly as a queue is acceptable.

Example 1:

Input

["MyStack", "push", "push", "top", "pop", "empty"]

[[], [1], [2], [], [], []]

Output

[null, null, null, 2, 2, false]

Example 2:

Explanation

MyStack myStack = new MyStack();

myStack.push(1);

myStack.push(2);

myStack.top(); // return 2

myStack.pop(); // return 2

myStack.empty(); // return False

Code

class MyStack:

    def __init__(self):
        

    def push(self, x: int) -> None:
        

    def pop(self) -> int:
        

    def top(self) -> int:
        

    def empty(self) -> bool:
        


# 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()

class MyStack:

    def __init__(self):
        

    def push(self, x: int) -> None:
        

    def pop(self) -> int:
        

    def top(self) -> int:
        

    def empty(self) -> bool:
        


# 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()