Two lc medium level questions, one minimum path problem using BFS, one data structrue design problem. Same questions as https://leetcode.com/discuss/interview-question/873993/imc-trading-new-grad-fast-track-interview-questions-from-hackerrank except for two small modifications:

1. The stack also has a function sum() that returns the sum of all values in the stack. Also needs to be in O(1) time.
2. There is a bishop on the board with fixed location (bi, bj). The knight mustn't be captured by the bishop.
   The implementation is not hard as long as you know how to perform BFS.