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.

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.

Thoughts:

Linkedlist style to record the head info,

record min history by iteratively comparing the min of the current value and history min

Code (Python)

class MinStack(object):

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.head = None

    def push(self, x):
        """
        :type x: int
        :rtype: void
        """
        if(self.head == None):
            self.head = Node(x, x);
        else:
            self.head = Node(x, min(self.head.min, x), self.head)

    def pop(self):
        """
        :rtype: void
        """
        self.head = self.head.next

    def top(self):
        """
        :rtype: int
        """
        return self.head.val

    def getMin(self):
        """
        :rtype: int
        """
        return self.head.min

class Node(object):

    def __init__(self, val, minval, next = None):
        self.val = val
        self.min = minval
        self.next = next


# Your MinStack object will be instantiated and called as such:
# obj = MinStack()
# obj.push(x)
# obj.pop()
# param_3 = obj.top()
# param_4 = obj.getMin()