采用二分法，当下标对应的值大于下标时，l = mid，否则，r = mid - 1

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