[NEW] Add solution for Mex Grid Construction

[ks-iitjmu]

🎯 Problem Information

• Problem Name: Mex Grid Construction
• CSES Link: https://cses.fi/problemset/task/3419
• Category: Introductory Problems
• Estimated Difficulty: Medium

📋 Problem Description

[Nitin2332]

hi @ks-iitjmu ,I would like to work on this issue and provide a solution for Mex Grid Construction. Please assign it to me.

[ks-iitjmu]

@Nitin2332 Assigned