They send me a piece of code to optimize. The original codes contain matrix manipulation and take more than one day to get the answer. I spent one night to optimize it to make the calculation time to about 0.1 sec. After two days received email rejection, without any explanation. BTW, The original code has a Time Complexity O(n^2) and space complexity O(n^2) too. I optimized both to be O(n). I don't think there is a better method exist! Overall, it's a waste my time.