二分法

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