# 155. Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

``````push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
getMin() -- Retrieve the minimum element in the stack.
``````

c

• code, one stack, save the min at each time at the second place, for e.g. input (1,3,-2) will become (1,1),(3,1)(-2,-2)
``````class MinStack:

def __init__(self):
"""
"""
self.stack= []

def push(self, x):
"""
:type x: int
:rtype: nothing
"""
if not self.stack:self.stack.append((x,x))
else:
self.stack.append((x,min(x,self.stack[-1][1])))

def pop(self):
"""
:rtype: nothing
"""
if self.stack: self.stack.pop()

def top(self):
"""
:rtype: int
"""
if self.stack: return self.stack[-1][0]
else: return None

def getMin(self):
"""
:rtype: int
"""
if self.stack: return self.stack[-1][1]
else: return None

``````
• c two stacks
``````class MinStack:

def __init__(self):
"""
"""
self.min_stack = []
self.stack = []

def push(self, x: int) -> None:
self.stack.append(x)
if not self.min_stack or x <= self.min_stack[-1]:
self.min_stack.append(x)

def pop(self) -> None:
ele = self.stack.pop()
if ele == self.min_stack[-1]:
self.min_stack.pop()
return ele

def top(self) -> int:
return self.stack[-1]

def getMin(self) -> int:
return self.min_stack[-1]
``````