class Solution {
public:
    int getMissingNumber(vector<int>& nums) {
        if(nums.empty())    return 0;
        int l=0;
        int r=nums.size()-1;
        while(l<r)
        {
            int mid=l+r>>1;
            if(nums[mid]!=mid)  r=mid;
            else l=mid+1;
        }
        if(nums[r]==r)   r++;
        return r;
    }

}