The typical 3-round interview process consists of the following steps.

1. Phone screen (behavioral only, discussed background). Roughly 30 minutes.
2. Video call/screenshare programming with one senior developer. Roughly 1 hour.
3. On-site interview at Proofpoint offices in Sunnyvale. Roughly 6 hours.

Write regex for any combination of a and then b where b can appear anytimes but a cannot appear consecutively. Don't use lookahead/lookback, only the *, ?, + characters and the union and grouping.

Given an input string and a second string called "alphabet" that consists of lettters, sort the input string such that the input string consists of the letters sorted by the 'alphabet' (and any characters in the input string not in the alphabet appear in the order they were given).

Find the most recent common ancestor of a binary search tree.

Given digits representing numbers to be pressed on the T9 keypad, return all possible strings that can be made.

Given a stream of integers, find the sliding window mean, product, max, and median for a given window size.