@@ -0,0 +1,435 @@
+class Solution %7B%0A    public int missingNumber(int%5B%5D x) %7B%0A        int len = x.length;%0A        int expect = len*(len+1) / 2;%0A        int sum = 0;%0A        for (int e : x) %7B%0A            sum += e;%0A        %7D%0A        return expect - sum;%0A    %7D%0A%7D%0A%0Apublic class missing_number %7B%0A    public static void main(String%5B%5D args) %7B%0A        Solution s = new Solution();%0A        int%5B%5D a = %7B0,1,3%7D;%0A        System.out.println(s.missingNumber(a));%0A    %7D%0A%7D%0A

add https://leetcode.com/problems/missing-number/