Bold Words in String

Given an array of keywords words and a string s, make all appearances of all keywords words[i] in s bold. Any letters between <b> and </b> tags become bold. Return s after adding the bold tags. The returned string should use the least number of tags possible, and the tags should form a valid combination.

"ab" appears at index 1, "bc" appears at index 2. Their marked ranges overlap into one bold region covering indices 1..3.