Given environment variables & string pattern which uses vairables, return final formated string by substituting environment variable values. Design Lib for this.

Input -
Environment variable -
"PYTHON" : "/lib/python/bin"
Pattern :
"/username/%PYTHON%"

Output -
"/username/lib/python/bin"

I was able to solve this but not able to complete the code.
Time complexity - O (n^2) where n is length of pattern & length of param

Environment varaible can have variables too which made it little complex.

Answer & Time complexity in the comment will be appreciated.