min-stack_155.py

# Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

# Implement the MinStack class:

# 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.
# You must implement a solution with O(1) time complexity for each function.


# 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
# ---------------------------------------Runtime 67 ms Beats 85.26% Memory 20.4 MB Beats 49.65%---------------------------------------


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

    def push(self, val: int) -> None:
        self.stack.append(val)
        if self.min >= val:
            self.prevMin.append(self.min)
            self.min = val

    def pop(self) -> None:
        val = self.stack.pop()
        if self.min == val:
            self.min = self.prevMin.pop()

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

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