added a couple of test cases for binarysearch for ranges
Added interview question on strings
Added alternative solution to character duplicate removal question
added matrix-based interview question
Added numbers-as-linkedlist sum interview question
Added interview question for stack with min() in O(1)
Added queue-with-two-stacks interview question
added another interview question
Added few tests for edge cases
generate all powersets
added test for edge case
generate all permutations of a string