lintcode-medium-Find the Missing Number

Given an array contains N numbers of 0 .. N, find which number doesn‘t exist in the array.

Given N = 3 and the array [0, 1, 3], return 2.

public class Solution {
    /**
     * @param nums: an array of integers
     * @return: an integer
     */
    public int findMissing(int[] nums) {
        // write your code here

        if(nums == null || nums.length == 0)
            return 0;

        int sum = 0;

        for(int i = 0; i < nums.length; i++){
            sum += nums[i];
        }

        int N = nums.length;

        return N * (N + 1) / 2 - sum;

    }
}