Added few useful algorithms

[sorohere]

Closes # <Do NOT delete this template> Description: Added three important ans useful algorithms. Maximum depth of a binary tree, Check if two trees are some or not and Merging two linked list.

Checklist

• [x] I've followed the Contributing guidelines provided in the repository.
• [x] I've made the changes which were demanded in the linked issue.
• [x] My code gave a clean output on debugging. (NO warnings/errors)