class Solution {
public:
    int getMissingNumber(vector<int>& nums) {
        //左边满足性质nums[i] == i,右边不满足，故而可以考虑二分
        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[r] == r) r++;
        return r;
    }
};