find max value in a stack by push and pop without changing the stack by python: wirte code

下面是一种实现方法，可以在不改变栈顺序的情况下查找栈中的最大值：

class MaxStack:
    def __init__(self):
        self.stack = []
        self.max_stack = []

    def push(self, value):
        self.stack.append(value)
        if not self.max_stack or value >= self.max_stack[-1]:
            self.max_stack.append(value)

    def pop(self):
        if self.stack:
            value = self.stack.pop()
            if value == self.max_stack[-1]:
                self.max_stack.pop()
            return value
        return None

    def max(self):
        if self.max_stack:
            return self.max_stack[-1]
        return None

使用方法：

stack = MaxStack()
stack.push(1)
stack.push(2)
stack.push(3)
print(stack.max()) # Output: 3
stack.pop()
print(stack.max()) # Output: 2