我的 Leetcode 程序出现了超出范围的错误

Question

So, I'm working on a relatively simple program on leetcode ( https://leetcode.com/problems/plus-one/ ). I'll copy the description below:
You are given a large integer represented as an integer array digits, where each digits[i] is the ith digit of the integer. The digits are ordered from most significant to least significant in left-to-right order. The large integer does not contain any leading 0's.

Increment the large integer by one and return the resulting array of digits.

Example: if digits = [1,2,3] then after digits = [1,2,4], because 123 + 1 = 124.

Anyways my code works for all the inputs I've tried except when the array consists of all 9's. I'm not sure why but I get an out of range error:

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 0) >= this->size() (which is 0)

I know that my code so far may not be the most optimal but I'd like to get it right my way before I attempt any sort of optimizing. I'll include my code below:

class Solution {
public:
    vector<int> plusOne(vector<int>& digits) 
    { 
        if(digits.at(digits.size()-1) < 9)
        {
            digits.at(digits.size()-1) += 1;
        }
        else
        {
            int zeroCount = 0;
            int index = 0;
            for(int i = digits.size()-1; i >= 0;--i)
            {
                if(digits.at(i) == 9)
                {
                    digits.pop_back();
                    zeroCount++;
                }
                else
                {
                    index = i;
                    break;
                }
            }
            if(digits.at(index) < 9)
            {
                digits.at(index) += 1;
                for(int i = 0; i < zeroCount; ++i)
                {
                    digits.push_back(0);
                }
            }
            else
            {
                digits.push_back(1);
                for(int i = 0; i < zeroCount; ++i)
                {
                    digits.push_back(0);
                }
            }
        }
     return digits;
    }
};

Answer 1

If all elements are 9, all elements will be removed by this part:

                if(digits.at(i) == 9)
                {
                    digits.pop_back();
                    zeroCount++;
                }

Therefore, the condition digits.at(index) < 9 becomes invalid after this operation.
This condition should be .digits.empty() && digits.at(index) < 9 to avoid this error.