LeetCode [1539] Kth Missing Positive Number

 1539. Kth Missing Positive Number

Easy

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.

 

Constraints:

• 1 <= arr.length <= 1000
• 1 <= arr[i] <= 1000
• 1 <= k <= 1000
• arr[i] < arr[j] for 1 <= i < j <= arr.length

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class Solution { public int findKthPositive(int[] arr, int k) { int n = arr.length; //arr[i-1] - i = missed //find max i s.t. arr[i-1] - i < k //return i+k int l = 0, r = n, mid = 0; while(l<=r){ mid = (l+r)/2; int missed = getMissed(arr, mid); if(missed<k && (mid==r || getMissed(arr, mid+1) >= k)) break; else if(missed<k) l = mid+1; else r = mid-1; } return mid+k; } int getMissed(int[] arr, int len){ if(len==0) return 0; else return arr[len-1] - len; } }