question3.py

#NB used python 3
# 3 Maximum In A stack

class MaxStack:
    def __init__(self):
        self.elements = []
        self.maximums = []
        
    def push(self, val):
        self.elements.append(val)
        if not self.maximums:
            self.maximums.append(val)
        else:
            self.maximums.append(max(self.maximums[-1], val))
            
    def pop(self):
        self.elements.pop()
        self.maximums.pop()
        
    def max(self):
        if self.maximums:
            return self.maximums[-1]
        return None
        
s = MaxStack()
s.push(1)
s.push(2)
s.push(3)
s.push(2)
print(s.max())
# 3
s.pop()
s.pop()
print (s.max())