Stack

class Stack:
    def __init__(self):
        self.s = []
        
    def push(self, item):
        self.s.append(item)
        
    def isEmpty(self):
        return len(self.s) == 0
    
    def pop(self):
        if self.isEmpty() == False:
            return self.s.pop(-1)	
        else:
            return None
    
    def peek(self):
        if self.isEmpty() == False:
            return self.s[-1]
        else:
            return None