Problem Challenge 3: Frequency Stack (hard)

Problem Statement

Design a class that simulates a Stack data structure, implementing the following two operations:

1. push(int num): Pushes the number ‘num’ on the stack.
2. pop(): Returns the most frequent number in the stack. If there is a tie, return the number which was pushed later.

Example:

After following push operations: push(1), push(2), push(3), push(2), push(1), push(2), push(5)

1. pop() should return 2, as it is the most frequent number
2. Next pop() should return 1
3. Next pop() should return 2

Constraints:

• 0 <= val <= 10⁹