Update and rename Missing Numbers.java to Missing Number.java

Author: varunu28

Easy/Missing Number.java

@@ -0,0 +1,6 @@
+class Solution {
+  public int missingNumber(int[] nums) {
+    return (nums.length * (nums.length + 1) / 2) - Arrays.stream(nums).boxed()
+        .reduce(0, Integer::sum);
+  }
+}