First interview: Easy/medium question. I got a string manipulation problem, and generally, that seems to be the trend.
Second interview: Medium/hard question. Generally more complex algorithms like graph search, recursion, or DP.
Read through Cracking the Coding Interview a few times. Plenty of review questions for data structures. Then start solving questions on CareerCup and CodeEval.
LeetCode Medium Problem about Graphs or DP
The following metrics were computed from 80 interview experiences for the Google Software Engineer role in London, England.
Google's interview process for their Software Engineer roles in London, England is extremely selective, failing the vast majority of engineers.
Candidates reported having very good feelings for Google's Software Engineer interview process in London, England.