class Solution {
public:
    int getMissingNumber(vector<int>& nums) {
        if (nums.empty()) return 0;

        int l = 0, r = nums.size() - 1;
        while (l < r){
            int mid = l + r >> 1;
            if (nums[mid] != mid) r = mid;
            else l = mid + 1;
        }

        if (nums[l] == l) l ++ ;
        return l;
    }
};