func missingNumber(_ nums: [Int]) -> Int {
    let count = nums.count
    // What's this all about?
    var sum = ((0 + count)*(count + 1)) / 2
    for i in 0..<count {
        sum -= nums[i]
    }
    return sum
}

let array = [0, 1, 3] // missing # is 2
missingNumber(array) // returns 2...HOW?!?