Find Kth Missing Positive Number

[Question]: Given an array arr of positive integers sorted in a strictly increasing order, and an integer k. Return the kth positive integer that is missing from this array.

Example 1:

Input: arr = [2,3,4,7,11], k = 5
Output: 9
Explanation: The missing positive integers are [1,5,6,8,9,10,12,13,...]. The 5th missing positive integer is 9.

Example 2:

Input: arr = [1,2,3,4], k = 2
Output: 6
Explanation: The missing positive integers are [5,6,7,...]. The 2nd missing positive integer is 6.

// TC: O(Nlog(N))
// SC: O(1)
func findKthPositive(_ arr: [Int], _ k: Int) -> Int {
    var left = 0
    var right = arr.count - 1
    while left <= right {
        let mid = left + (right-left)/2
        if arr[mid] - mid - 1 < k {
            left = mid + 1
        } else {
            right = mid - 1
        }
    }
    return left + k
}