solves single number

Author: anishLearnsToCode

src/SingleNumber.java

@@ -0,0 +1,20 @@
+import java.util.HashSet;
+import java.util.Set;
+
+public class SingleNumber {
+    public int singleNumber(int[] array) {
+        Set<Integer> numbers = new HashSet<>();
+        for (int element : array) {
+            if (numbers.contains(element)) {
+                numbers.remove(element);
+            } else {
+                numbers.add(element);
+            }
+        }
+
+        for (int number : numbers) {
+            return number;
+        }
+        return -1;
+    }
+}