ZhouYang1993

class Solution:
    def backspaceCompare(self, S: str, T: str) -> bool:
        def get_stack(string: str):
            stack = []
            for s in string:
                if s != '#':
                    stack.append(s)
                else:
                    if stack:
                        stack.pop()

ZhouYang1993

class Solution:
    def removeDuplicates(self, S: str) -> str:
        stack = []
        for char in S:
            if (not stack) or (stack[-1]!=char):
                stack.append(char)
            else:
                while stack and stack[-1] == char:
                    stack.pop()
        return ''.join(stack)

ZhouYang1993

class Solution:
    def removeDuplicates(self, S: str) -> str:
        stack = []
        for char in S:
            if (not stack) or (stack[-1]!=char):
                stack.append(char)
            else:
                while stack and stack[-1] == char:
                    stack.pop()
        return ''.join(stack)

ZhouYang1993

class Solution:
    def largestRectangleArea(self, heights: List[int]) -> int:
        ret = 0
        mono_stack = []
        heights.append(0)
        for i, v in enumerate(heights):
            while mono_stack and heights[mono_stack[-1]] > v:
                height = heights[mono_stack[-1]]
                length = i - mono_stack[-1]
                ret = max(ret, height * length)

ZhouYang1993

class Solution:
    def largestRectangleArea(self, heights: List[int]) -> int:
        ret = 0
        mono_stack = []
        heights.append(0)
        for i, v in enumerate(heights):
            while mono_stack and heights[mono_stack[-1]] > v:
                height = heights[mono_stack.pop()]
                if mono_stack:
                    length = i - mono_stack[-1]-1

ZhouYang1993

class Solution:
    def trap(self, heights: List[int]) -> int:
        stack = []  # a decreasing stack
        total = 0
        for i, v in enumerate(heights):
            while stack and heights[stack[-1]] < v:
                popped_idx = stack.pop()
                if not stack:
                    break
                height = min(heights[stack[-1]], v) - heights[popped_idx]

ZhouYang1993

#### example 1
class Animal(ABC):
    @abstractmethod
    def move(self):
        pass
a = Animal() 
# TypeError: Can't instantiate abstract class Animal with abstract methods move


#### example 2

ZhouYang1993

from abc import ABC, abstractmethod
### Example 1
class Animal(ABC):
    @abstractmethod
    def move(self):
        pass
a = Animal() 
# TypeError: Can't instantiate abstract class Animal with abstract methods move

### Example 2

ZhouYang1993

from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def move(self):
        print('Animal moves')
        pass

class Cat(Animal):
    def move(self):

ZhouYang1993

from abc import ABC, abstractmethod

class Animal(ABC):
    @abstractmethod
    def move(self):
        print('Animal moves')

class Cat(Animal):
    def move(self):
        super().move()