Serveral months ago, I practiced serveral binary leetcode questions, and I was confused about the following several things: I didn’t pay much attentions to it then, I thought I will get the hang of it after serveral practices. However after serveral practice, I still confused in binary search problems. Latest week an article mentions binary search at https://leetcode.wang/. Little searching, I found more information about this such as lower bound, upper bound, equal-range, open/close range, middle point etc.