10031. Smallest Missing Integer Greater Than Sequential Prefix Sum

You are given a 0-indexed array of integers nums.

A prefix nums[0..i] is sequential if, for all 1 <= j <= i, nums[j] = nums[j - 1] + 1. In particular, the prefix consisting only of nums[0] is sequential.

Return the smallest integer x missing from nums such that x is greater than or equal to the sum of the longest sequential prefix.

class Solution {
public:
    int missingInteger(vector<int>& nums) {
        int res = nums[0];
        for(int i = 1; i < nums.size(); i++) {
            if(nums[i] == nums[i-1] + 1) res += nums[i];
            else break;
        }
        unordered_set<int> us(begin(nums), end(nums));
        while(us.count(res)) res += 1;
        return res;
    }
};