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) {
                l = mid + 1;
            } else {
                r = mid;
            }
        }

        if (nums[r] == r) {
            return nums.size();
        }
        return r;
    }
};