> For the complete documentation index, see [llms.txt](https://nataliekung.gitbook.io/ladder_code/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://nataliekung.gitbook.io/ladder_code/meta-2025/1539.-kth-missing-positive-number.md).
# 1539. Kth Missing Positive Number
number
Given an array `arr` of positive integers sorted in a **strictly increasing order**, and an integer `k`.
Return *the* `k``th` ***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.
**Constraints:**
* `1 <= arr.length <= 1000`
* `1 <= arr[i] <= 1000`
* `1 <= k <= 1000`
* `arr[i] < arr[j]` for `1 <= i < j <= arr.length`
分析
直接遍历数组，一边计算当前缺失了几个数字，一边找。
```
class Solution:
def findKthPositive(self, arr: List[int], k: int) -> int:
missing = 0
current = 1
i = 0
while True:
if i < len(arr) and arr[i] == current:
i += 1
else:
missing += 1
if missing == k:
return current
current += 1
```