Lets see the first 6 operations in "RGB" string.

RGB -> GRB -> GBR -> BGR -> BRG -> RBG -> RGB

so we can see the string repeats itself in every 6 operations. Applying 6 operations is equivalent to 6%6=0 operations. applying 7 operations is equivalent to 7%6=1 operations and so on.

That means we dont need to do all K operations. Instead we can find the answer in just K%6 operations.

time complexity: O(1)