diff --git a/src/main/java/leetcode/medium/187. Repeated DNA Sequences.java b/src/main/java/leetcode/medium/187. Repeated DNA Sequences.java
new file mode 100644
index 0000000..2cb98af
--- /dev/null
+++ b/src/main/java/leetcode/medium/187. Repeated DNA Sequences.java	
@@ -0,0 +1,37 @@
+class Solution {
+    public List<String> findRepeatedDnaSequences(String s) {
+          List<String> result=new ArrayList<String>();
+          int len = s.length();
+          if(len<10){
+            return result;
+          }
+
+          Map<Character, Integer> map= new HashMap<Character, Integer>();
+          map.put('A',0);
+          map.put('C',1);
+          map.put('G',2);
+          map.put('T',3);
+
+          Set<Integer> temp=new HashSet<Integer>();
+          Set<Integer> added = new HashSet<Integer>();
+          int hash=0;
+          for(int i=0; i<len;i++){
+            if(i<9){
+                //each ACGT fir in 2 bits so leftShift 2
+                hash=(hash<<2)+map.get(s.charAt(i));
+            }else{
+                hash=(hash<<2)+map.get(s.charAt(i));
+                //male length of hash to be 20
+                hash = hash & (1<<20) -1;
+                if(temp.contains(hash) && !added.contains(hash)){
+                    result.add(s.substring(i - 9, i+1));
+                    added.add(hash);//track added
+
+                }else{
+                    temp.add(hash);
+                }
+            }
+          }
+          return result;
+    }
+}