Repeated DNA Sequences (LeetCode Bit Manipulation)

Question: All DNA is composed of a series of nucleotides abbreviated as A, C, G, and T, for example: "ACGAATTCCG". When studying DNA, it is sometimes useful to identify repeated sequences within the DNA.
Write a function to find all the 10-letter-long sequences (substrings) that occur more than once in a DNA molecule.
For example,
Given s = "AAAAACCCCCAAAAACCCCCCAAAAAGGGTTT",
Return:
["AAAAACCCCC", "CCCCCAAAAA"].

Idea: Create a HashMap<bit representation of substring, appeared times>. For each 10 letter window, calculate its bit representation as the key. Then slide the window from left to right, if the key shows up exactly twice, append the substring to the result.

Time: O(n) Space: O(n)

Code:

 public class Solution {  
   public List<String> findRepeatedDnaSequences(String s) {  
     List<String> result=new ArrayList<String>();  
     HashMap<Integer,Integer> computed=new HashMap<Integer,Integer>();  
     for(int i=0;i<=s.length()-10;i++)  
     {  
       String sub=s.substring(i,i+10);  
       int key=getKey(sub);  
       if(computed.containsKey(key))  
       {  
         computed.put(key,computed.get(key)+1);  
         if(computed.get(key)==2)  
           result.add(sub);  
       }  
       else  
         computed.put(key,1);  
     }  
     return result;  
   }  
   public int getKey(String s)  
   {  
     int result=0;  
     for(int i=s.length()-1;i>=0;i--)  
     {  
       int b=0;  
       switch(s.charAt(i))  
       {  
         case 'A':  
           b|=0;  
           break;  
         case 'T':  
           b|=1;  
           break;  
         case 'G':  
           b|=2;  
           break;  
         case 'C':  
           b|=3;  
           break;  
       }  
       result=b|result;  
       result=result<<2;  
     }  
     return result;  
   }  
 }