java1015 leetcode1 twosum approach2 Map-doc generic PESC type argument/(? extends | super %bounded) parameterized type

 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