Min Stack Problem

Problem Statement

Design a min stack that allows the following operations:

• push(int x)
  : Pushes element
  x
  onto the stack.
• pop()
  : Removes and returns the top element from the stack. If the stack is empty, it returns
  -1
  .
• top()
  : Returns the top element from the stack without removing it.
• getMin()
  : Returns the minimum element in the stack.

Rules and Constraints

• The
  push
  and
  pop
  operations should take constant time.
• The
  top
  and
  getMin
  operations should take constant time.
• The total memory used should be O(n) where n is the number of elements pushed into the stack.

Notes

The min stack should handle the following edge cases:

• When the stack is empty,
  pop()
  should return
  -1
  .
• When no elements are pushed into the stack,
  getMin()
  should return a meaningful value (i.e. not a default value or error).
• When the only element pushed into the stack is popped out, the
  getMin()
  operation should return the same result as when the element was first pushed into the stack.

The min stack should maintain the order in which elements were pushed onto the stack.

Example

Input: {"operations":["push","push","getMin"]} Output: [null,null,-3]