Convert the strings into character arrays and then add the first string in Hashtable by looping through all characters of array and adding them in the Hashtable. Then in another loop check if characters of the second string are contained in Hastable. Complexity would be O(length of larger string).... I think it should work...

Actually you need to take care of the duplicates otherwise you will get duplicates. The following code provides correct answer. public class commoncharnodup { private static void noDup(String in1,String in2) { Hashtable inhash=new Hashtable(); StringBuilder commonChar=new StringBuilder(); for(int i=0;i