Add Solution.java for 0268.Missing Number

Author: huaxu1024

solution/0268.Missing Number/Solution.java

@@ -0,0 +1,9 @@
+class Solution {
+    public int missingNumber(int[] nums) {
+        int res = nums.length;
+        for(int i = 0; i < nums.length; i++){
+            res += i - nums[i];
+        }
+        return res;
+    }
+}