minStack.py

# 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.

# Example:
# MinStack minStack = new MinStack();
# minStack.push(-2);
# minStack.push(0);
# minStack.push(-3);
# minStack.getMin();   --> Returns -3.
# minStack.pop();
# minStack.top();      --> Returns 0.
# minStack.getMin();   --> Returns -2.

# SOLUTION ONE
class MinStack(object):

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.stack = []
        

    def push(self, x):
        """
        :type x: int
        :rtype: void
        """
        self.stack.append(x)
        

    def pop(self):
        """
        :rtype: void
        """
        self.stack.pop()

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

    def getMin(self):
        """
        :rtype: int
        """
        return min(self.stack)
        
# A BETTER SOLUTION
class MinStack(object):

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.stack = []
        self.min = []
        

    def push(self, x):
        """
        :type x: int
        :rtype: void
        """
        self.stack.append(x)
        if(len(self.min) > 0):
            self.min.append(min([x, self.min[-1]]))
        else:
            self.min.append(x)
        

    def pop(self):
        """
        :rtype: void
        """
        self.stack.pop()
        self.min.pop()

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

    def getMin(self):
        """
        :rtype: int
        """
        return self.min[-1]

# ANOTHER BETTER SOLUTION
class MinStack:

    def __init__(self):
        self.q = []

# @param x, an integer
# @return an integer
    def push(self, x):
        curMin = self.getMin()
        if curMin == None or x < curMin:
            curMin = x
        self.q.append((x, curMin));

# @return nothing
    def pop(self):
        self.q.pop()


# @return an integer
    def top(self):
        if len(self.q) == 0:
            return None
        else:
            return self.q[len(self.q) - 1][0]


# @return an integer
    def getMin(self):
        if len(self.q) == 0:
            return None
        else:
            return self.q[len(self.q) - 1][1]