Greatest Common Divisor of Strings The brute force way would be considering all the prefixes of one of the strings and simulate to see if that is a candidate answer.
N-th Tribonacci Number Since each number only depends on the last three numbers, we can pre compute the results in an array and return the results in ({\Omicron(1)}) for all the queries.
Maximum Sum Circular Subarray The maximum sum problem can be solved using the well known Kadane’s algo. But this problem becomes a bit special because of the circular element.
Subarray Sums Divisible by K Well known sub-array divisibility problem. Only catch was that the numbers could be negative. So don’t forget to handle modulus for negative values.