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class Stack(object):
def __init__(self):
self.items = []
def __repr__(self):
return self.items.__repr__()
def push(self, x):
self.items.append(x)
def pop(self):
if not self.items:
return None
return self.items.pop()
def peek(self):
if not self.items:
return None
return self.items[-1]
def from_list(xs):
result = Stack()
for x in xs:
result.push(x)
return result
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