Solved FirstCommonAncestor, SiblingListsOfTree, ArraySumChallenge, most
of PathSumTree, added new puzzles (not solved yet)
Merge branch 'master' of https://github.com/PremjitAdhikary/practice
Solved IntersectingRectangles, PalindromeString, PascalsTriangle,
RemoveCharsInString, FibonacciChallenge, ReverseList, updated
PathSumTree, UniqueCharsInString, added puzzles
Added design Patterns, Singleton, Adapter, Observer, Mediator