Assign a new vector as the size is length + 1. Because if the original is 9999...., the it will have one digit more.

1 class Solution { 2 public: 3     vector
     
       plusOne(vector
      
        &digits) { 4         int len = digits.size(), carry = 0; 5         if (digits.size() == 0) return vector
       
         (); 6         vector
        
          result(len+1, 0); 7         digits[len-1]++; 8         for (int i = len-1; i >= 0; i--) { 9             result[i+1] = digits[i] + carry;10             carry = result[i+1]/10;11             result[i+1] %= 10;12         }13         if (carry) result[0] = 1;14         else result.erase(result.begin());15         return result;16     }17 };