MongoDB New/Recent Grad Software Engineering interview questions

One phone screen. One technical phone interview coding on google docs. How would you write DB write / access functions to make sure data is not modified by threads. Parse mathematical expression given as chars in array ['1', '+', '2,' '/', '4']. Four onsite interviews over the course of 4 hours asking a variety of questions. Write algorithm to find loop in graph, write C code to convert integer to string.