class Solution {
public:
    int getNumberSameAsIndex(vector<int>& nums) {
        //二分，单调性序列，nums[i] - i,我们需要找到一个nums[i] - i == 0的数
        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;   //要加等号，Chech条件
            else l = mid + 1;
        }
        //if(r == nums.size() - 1) return ++r;
        if(nums[r] - r == 0) return r;
        return -1;
    }
};