Leecode_Missing number

The Problem:

Given an array containing n distinct numbers taken from 

0, 1, 2, ..., n

,
find the one that is missing from the array.

For example,

Given nums = 

[0, 1, 3]

 return 

2

.

Note:

Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?
My wrong solution:  Dichotomy
Right Solution: the sum of size+1 numbers - the sum of the array = the missing number

public class Solution {

    public int missingNumber(int[] nums) {

        int size = nums.length;

        if(size < 1)

            return 0;

        else

        {

            int n1 = (size+1)*size/2;

            int n2 = 0;

            for(int i = 0; i<size; i++)

                n2 = n2+nums[i];

            return n1-n2;

        }

    }

}