leetcode Plus One

class Solution {
public:
vector<int> plusOne(vector<int> &digits) {
int size = digits.size();
if(size == 0){
digits.push_back(1);
return digits;
}
int idx = size - 1, carry = 1;
//digits[idx] += 1;
/*critical part*/
while(idx >= 0){
digits[idx] += carry;
carry = digits[idx] / 10;
digits[idx] %= 10;
idx--;
}
while(carry > 0){
digits.insert(digits.begin(), carry % 10);
carry /= 10;
}
return digits;
}
};