题目链接：https://www.acwing.com/problem/content/65/
题目如下：

class Solution {
public:
    int getNumberSameAsIndex(vector<int>& nums) {
        int low=0,high=nums.size()-1;
        while(low<=high){
            int mid=low+(high-mid)/2;
            
            if(nums[mid]==mid) return mid;
            else if(nums[mid]<mid) low=mid+1;
            else if(nums[mid]>mid) high=mid-1;
        }
        
        return -1;
    }
};