## DEV Community

Abhishek Chaudhary

Posted on

# 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()
``````