Build a Guess the Coin Game (Challenge)

# Introduction

In this challenge, we will help Tom complete a coin guessing game. The game has 9 cups and the system selects 3 cups randomly to put coins in. The player needs to enter a string with cup numbers to make their guesses, and the system will open the corresponding cups. The goal is to complete the functions `findNum` and `randomCoin` to achieve the game's functionality.