268. Missing Number

// LeetCode URL: https://leetcode.com/problems/missing-number/

/**
 * Sum all the numbers and subtract the sum from n * (n+1) / 2
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(1)
 *
 * N = Length of the input array.
 */
class Solution {
    public int missingNumber(int[] nums) {
        if (nums == null || nums.length == 0) {
            return -1;
        }

        int sum = 0;
        int len = nums.length;
        for (int num : nums) {
            sum += num;
        }

        return len * (len + 1) / 2 - sum;
    }
}