Added RemoveDuplicateString problem

src/codingquestions/leetcode/RemoveDuplicateFromString.java

@@ -0,0 +1,40 @@
+import java.util.*;
+
+class RemoveDuplicateFromString{
+
+    public static String removeDuplicate(String s, int idx, StringBuilder str, boolean[] map){
+        //base case
+        if(idx == s.length()-1){
+            return str.toString();
+        }
+        char ch = s.charAt(idx);
+        if(map[ch-'a'] == true){
+            removeDuplicate(s, idx+1, str, map);
+        } else {
+            map[ch-'a'] = true;
+            removeDuplicate(s, idx+1, str.append(ch), map);
+        }
+        return str.toString();
+    }
+
+    public static void main(String[] args){
+
+        String s = "appnacollege";
+        //Solution 1 using recursion
+        System.err.println(removeDuplicate(s, 0, new StringBuilder(""), new boolean[26]));
+
+
+        //Solution 2 using Hashset
+        HashSet<Character> ch = new HashSet<>();
+        StringBuilder unique = new StringBuilder();
+        for(int i=0; i<s.length(); i++){
+            char letter = s.charAt(i);
+            if(!ch.contains(letter)){
+                ch.add(letter);
+                unique.append(letter);
+            }
+        }
+        String ans = unique.toString();
+        System.err.println(ans);
+    }
+}