BlackRock Software Engineer interview questions

First technical interview: first they asked to find the number of "e" in string immediate in any language. Then how to insert in linked list. Then how to sort a linked list. Then few maths problem solving questions Second technical interview: what is big data, python, AWS, my projects again few problem solving. Third hr interview: tell me about yourself. Did you give IIT exam. About my family. Few behavorial questions and do u wanna ask anything.

How do you reduce time complexities from O(n2) to O(n) and then to O(logn)?