LeetCode： Kth Missing Positive Number

Question

Given an array arr of positive integers sorted in a strictly increasing order, and an integer k.

Find 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.

Solution#1

def find_kth_positive(arr, k)
  fetch_number = ((1..arr.max).to_a - arr).fetch(k-1, nil)

  if fetch_number.nil?
    # the maximum point for searching missing number shall add k value
    ((1..arr.max+k).to_a - arr).fetch(k-1)
  else
    ((1..arr.max).to_a - arr).fetch(k-1)
  end
end

Solution#2

def find_kth_positive(arr, k)
  left = 0
  right = arr.length - 1

  while left <= right
    mid = (left + right) >> 1
    if arr[mid] - mid - 1 < k
      left = mid + 1
    else
      right = mid - 1
    end
  end
  
  left + k
end