Go Perm Missing Element
func permMissingElement(a []int) int {
	sorted := append([]int(nil), a...)
	sort.Ints(sorted)

	for k, v := range sorted {
		if v != k+1 {
			return k + 1
		}
	}

	return len(sorted) + 1
}

This uses the expected sum of 1..N+1 and subtracts the actual sum to find the missing value.