Add solution articles

[sahilbansal17]

Problems

• Binary Search Tree Checker
• Find K closest elements
• Populating Next Right pointers
• Diameter of Generic Trees
• Game of Life

General

• Update articles for existing ones from text to markdown format.

Guidelines:

1. It has to be in markdown format. Consider this one as an example.
2. Try to include some hints as in the one above and write in a language which is easy to understand.
3. We do not wish to give the articles already there on gfg or leetcode etc. The articles should be such that it guides the reader to come to a solution themselves by getting enough hints. It should help them know more about our thought process in general.