min_stack.py

"""
https://leetcode.com/problems/min-stack/

Design a stack class that supports the push, pop, top, and getMin operations.

MinStack() initializes the stack object.
void push(int val) pushes the element val onto the stack.
void pop() removes the element on the top of the stack.
int top() gets the top element of the stack.
int getMin() retrieves the minimum element in the stack.
Each function should run in O(1) time.

Example 1:
Input: ["MinStack", "push", 1, "push", 2, "push", 0, "getMin", "pop", "top", "getMin"]
Output: [null,null,null,null,0,null,2,1]

Explanation:
MinStack minStack = new MinStack();
minStack.push(1);
minStack.push(2);
minStack.push(0);
minStack.getMin(); // return 0
minStack.pop();
minStack.top();    // return 2
minStack.getMin(); // return 1

Constraints:
-2^31 <= val <= 2^31 - 1.
pop, top and getMin will always be called on non-empty stacks.
"""

class MinStack:
    def __init__(self):
        self.min = float("inf")
        self.stack = []

    def push(self, val: int) -> None:
        if not self.stack:
            self.stack.append(0)
            self.min = val
        else:
            self.stack.append(val - self.min)
            if val < self.min:
                self.min = val

    def pop(self) -> None:
        if not self.stack:
            return

        pop = self.stack.pop()

        if pop < 0:
            self.min = self.min - pop

    def top(self) -> int:
        if not self.stack:
            return float('inf')
        top = self.stack[-1]
        if top > 0:
            return top + self.min
        else:
            return self.min

    def getMin(self) -> int:
        return self.min