//注意处理数组长度为0，1，2的情况
class Solution {
public:
    int getNumberSameAsIndex(vector<int>& nums){
        //处理空数组        
        if (nums.empty()) return -1;
        int l = -1;
        int r = nums.size();
        while(l + 1 != r){
            int mid = (l + r) >> 1;
            if(nums[mid] >= mid){
                r = mid;
            }else{
                l = mid;
            }
        }
        //二分到最后mid = r，所以直接将r作为返回值，找不到则返回-1
        if (nums[r] == r){
            return r;
        }else{
            return -1;
        }
    }
};