二分法

class Solution {
public:
    int getMissingNumber(vector<int>& nums) {
        int l = 0;
        int 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) r++; // 特判是n的情况
        return r;
    }
};