2195. Append K Integers With Minimal Sum

You are given an integer array nums and an integer k. Append k unique positive integers that do not appear in nums to nums such that the resulting total sum is minimum.

Return the sum of the k integers appended to nums

class Solution {
public:
    long long minimalKSum(vector<int>& nums, int k) {
        set<long long> num(nums.begin(), nums.end());
        long long find = 1, prev = 0;
        long long sum = 0;
        
        for(long long n : num) {
            if(find == n) find++;
            else {
                if(n - 1 - prev <= k) {
                    sum += (n) * (n-1) / 2 - (prev) * (prev + 1) / 2;
                    k -= (n - 1 - prev);
                } else {
                    sum += (prev + k) * (prev + k + 1) / 2 - (prev) * (prev + 1) / 2;
                    k = 0;
                }
                find = n + 1;
            }
            prev = n;
            if(!k) break;
        }
        if(k)
            sum += (prev + k) * (prev + k + 1) / 2 - (prev) * (prev + 1) / 2;
        return sum;
    }
};