4 questions on Hackerrank, 60 min.

1. Partitioning array: https://nataliekung.gitbook.io/ladder_code/twitter-oa/partitioning-array
2. Paint house: https://nataliekung.gitbook.io/ladder_code/twitter-oa/paint-house
3. Palindrom substring: https://nataliekung.gitbook.io/ladder_code/twitter-oa/palindromic-substrings
4. Twitter user network: given a matrix n * n, where the (i, j) entry is either 1 or 0, and a 1 represents that user i follows user j. If i follows j or j follows i, they are in the same group. Return the total number of groups.