Notebook

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 1:

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Explanation
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2

Constraints:

Methods pop, top and getMin operations will always be called on non-empty stacks.

Source

In [ ]:

# 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()

Code

In [ ]:

class MinStack:

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

    def push(self, n):
        """return none
        TRICK: each node in the stack has a minimum value"""
        if len(self.stack) == 0:
            self.stack.append((n, n))
        else:
            self.stack.append((n, min(n, self.stack[-1][1])))

    def pop(self):
        """return none."""
        return self.stack.pop()[0]  #return element

    def top(self):
        """return int."""
        return self.stack[-1][0]   #return element

    def getMin(self):
        """return int."""
        return self.stack[-1][1]   #return min value associated with element