class Solution {
public:
    int getNumberSameAsIndex(vector<int>& nums) {
        //依据数列性质找nums[i] - i == 0的数值即为所求，因为左边nums[i] - i <=0,右边大于0
        //注意chech函数中等号成立的条件有没有等号哦
        if(nums.empty()) return -1;

        int l = 0,r = nums.size() - 1;
        while(l < r){
            int mid = l + r >> 1;
            if(nums[mid] - mid >= 0) r = mid;
            else l = mid + 1;
        }

        if(nums[r] == r) return r;
        return -1;
    }
};