Question

I’ve an array of string, I want to find the duplicate strings in the array and want to make the duplicates null by using HashMap with a good time complexity.

Answer 1

Sounds like you want to use a Set. This clears all duplicate entries, but you can also just create an array which has the unique entries (and no null values)

String[] array =
Set<String> found = new LinkedHashSet<String>();
for(int i=0;i<array.length;i++)
   if(!found.add(array[i]))
       array[i] = null;

// just the entries without duplicates.
String[] unique = found.toArray(new String[found.size()]);