Question

I have following problem:

• I have a very long string String str = "abcdefghiart----"
• Now I want to iterate over string and want to find the first duplicate character in string
• This I can achieve with applying two for loops to search first duplicate

• This approach works fine but iteration complexity will be very high in case string very large and only last two characters are duplicate

  Now I want to minimize the complexity and optimize this code. I can use foreach loop too to iterate but still it will be two foreach loop. I don’t want to use any system library. Can someone help me out on this?

Answer 1

Here’s a method to find the first duplicate character in a string.

 public static String findFirstDuplicate(String string){

                 for(int i=0;i<string.length()-1; i++){
                     String c=string.charAt(i)+"";
                     if(string.indexOf(c, i+1)>-1)
                         return  c;
                 }

                 return null;
  }

AS for optimization, you can optimise it by customizing Merge Sort to find duplicate value and terminate when a duplicate is found. It is a bit complicated to explain here but you’ll be able to find the first duplicate in O(nlogn) at worst case using modified merge sort algorithm.