Min Stack

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.

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:

• -231 <= val <= 231 - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * 104 calls will be made to push, pop, top, and getMin.

SOLUTION:

import heapq

# class MinStack:

#     def __init__(self):
#         self.stack = []
#         self.heap = []
#         self.deleted = {}

#     def push(self, val: int) -> None:
#         heapq.heappush(self.heap, val)
#         self.stack.append(val)

#     def pop(self) -> None:
#         popped = self.stack.pop()
#         self.deleted[popped] = self.deleted.get(popped, 0) + 1

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

#     def getMin(self) -> int:
#         curr = self.heap[0]
#         while curr in self.deleted:
#             heapq.heappop(self.heap)
#             self.deleted[curr] -= 1
#             if self.deleted[curr] == 0:
#                 del self.deleted[curr]
#             curr = self.heap[0]
#         return curr

class MinStack:

    def __init__(self):
        self.stack = []

    def push(self, val: int) -> None:
        self.stack.append((val, min(self.stack[-1][1], val) if len(self.stack) > 0 else val))

    def pop(self) -> None:
        return self.stack.pop()[0]

    def top(self) -> int:
        return self.stack[-1][0]

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


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