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