Missing Number

I used sort.Ints(nums) to avoid implementing mergesort from scratch, yeah... kind of a cheat, but I wanted to focus on the logic instead. Once the array is sorted, my idea was simple: as I iterate through the array, each number should match its index. If it doesn't, that's the missing number. This approach passed 2 out of the 3 initial test cases, but it failed on the [0, 1] case, which should return 2. That’s when I realized if you make it through the entire array without finding a mismatch, then the missing number has to be n—the length of the array. So the final return case should be len(nums).

func missingNumber(nums []int) int {
  sort.Ints(nums)

  for i,num := range nums {
    if i != num {
        return i
    }
  }
  return len(nums)
}