225.mystack.py

# Implement a last-in-first-out (LIFO) stack using only two queues. The
# implemented stack should support all the functions of a normal stack (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 that 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 using only one queue?
#
#  Related Topics Stack Design Queue 👍 5730 👎 1173


# leetcode submit region begin(Prohibit modification and deletion)
class MyStack:

    def __init__(self):
        self._stack = []

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

    def pop(self) -> int:
        return self._stack.pop()

    def top(self) -> int:
        return self._stack[-1]

    def empty(self) -> bool:
        return self._stack == []


# 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()
# leetcode submit region end(Prohibit modification and deletion)