Meta Internship interview questions

Coding: Standard question, check sites like https://www.geeksforgeeks.org Research: my experience and my research projects

I had one interview- I was asked to right an algorithm that got a submatrix from a mXn matrix with 0/1 and needs to return the number of 1's in the matrix. at first I wrote an algorithm that runs in o(m) and that I was asked to improve it to O(1)