These array and string problems come from [[u:/roadmaps/pattern-22|Pattern ]]. Solve all of them before moving on.
Start with Two Sum and Product of Array Except Self. These are Amazon phone screen staples. Get comfortable writing them in under minutes.
Then work through Merge Intervals and Trapping Rain Water. Trapping Rain Water is one of Amazon's favorite hard problems. The linked walkthroughs break down every step.