Min Stack with O(1) Operations

Write a C++ program to design a stack that supports push, pop, top, and retrieving the minimum element in constant time. Implement the MinStack class with the following operations:

Operations

  
MinStack(): initializes the stack object.
  
void push(int val): pushes 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

Example 2

  
Input: ["MinStack","push","push","getMin","push","getMin","pop","getMin"]
[[],[5],[3],[],[1],[],[],[]]
  
Output: [null,null,null,3,null,1,null,3]
  
Explanation: 

  
MinStack minStack = new MinStack();
  
minStack.push(5);
  
minStack.push(3);
  
minStack.getMin(); // return 3
  
minStack.push(1);
  
minStack.getMin(); // return 1
  
minStack.pop();
  
minStack.getMin(); // return 3

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
  
Time Complexity: O(1) for all operations
  
Space Complexity: O(n)