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.

Python

 
class MinStack(object):
    def __init__(self):
        """
        initialize your data structure here.
        """
        self.min_stack = []
        self.min_num_stack = []

    def push(self, x):
        """
        :type x: int
        :rtype: void
        """
        if len(self.min_num_stack) == 0 or x <= self.min_num_stack[-1]:
            self.min_num_stack.append(x)
        self.min_stack.insert(0, x)

    def pop(self):
        """
        :rtype: void
        """
        if self.min_stack[0] == self.min_num_stack[-1]:
            self.min_num_stack.pop()
        del self.min_stack[0]

    def top(self):
        """
        :rtype: int
        """
        return self.min_stack[0]

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