Solution: Problem Challenge 3: Frequency Stack

Grokking the Coding Interview: Patterns for Coding Questions

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Problem Statement

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

push(int num): Pushes the number ‘num’ on the stack.
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 <= 10<sup>9</sup>

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