0021 - 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

• Use two parallel stacks - one for actual values and one for tracking minimums.
• When pushing, add value to main stack and add min(new_value, current_min) to min stack.
• When popping, remove from both stacks together, ensuring min stack always reflects minimum of remaining elements.

In [5]:

class MinStack:
    def __init__(self):
        self.stack=[]
        self.minstack=[]
        
    def push(self,val):
        self.stack.append(val)
        #min of val or top of minstack(if stack is not empty)
        val=min(val, self.minstack[-1] if self.minstack else val)
        self.minstack.append(val)
        
    def pop(self):
        self.stack.pop()
        self.minstack.pop()
        
    def top(self):
        return self.stack[-1]
    
    def getmin(self):
        return self.minstack[-1]