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

  • push(x) -- Push element x onto stack.
  • pop() -- Removes the element on top of the stack.
  • top() -- Get the top element.
  • getMin() -- Retrieve the minimum element in the stack.

Example:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

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class MinStack(object):

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.stack = []
        self.min_val = None
        self.min_idx = None
        

    def push(self, x):
        """
        :type x: int
        :rtype: void
        """
        if self.min_val is None:
            self.min_val = x
            self.min_idx = 0 # exclude repulicated values
            self.stack.append(x)
        elif x < self.min_val:
            self.min_val = x
            self.stack.append(x)
            self.min_idx = len(self.stack) - 1
        else:
            self.stack.append(x)
            
        

    def pop(self):
        """
        :rtype: void
        """
        # is poped value is min_val
        if self.min_idx == len(self.stack) -1: # Probability is 1/n
            new_stack = self.stack[:-1]
            if new_stack: # O(n) some times 
                self.min_val = min(new_stack)
                self.min_idx = new_stack.index(self.min_val)
            else:
                self.min_val = None
                self.min_idx = None
        self.stack.pop()
        if len(self.stack) == 0:
            self.min_val = None
            self.min_idx = None
        
                
        

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

    def getMin(self):
        """
        :rtype: int
        """
        return self.min_val
        


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