leetcode1 twosum approach2:
https://leetcode.com/problems/two-sum/solution/#
Map-doc->hashmap:
https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html
generic PESC:
type argument/(? extends | super %bounded) parameterized type:
https://docs.oracle.com/javase/tutorial/java/generics/genTypeInference.html
https://stackoverflow.com/questions/1759549/java-generics-multiple-generic-parameters#
https://stackoverflow.com/questions/18575630/how-generic-subtype-and-wildcards-work-in-collections-copy