A function to add two 50-digit numbers and return it

string addBigNumbers(string n1, string n2){
  string result = ""; 
  int len1 = n1.size(); 
  int len2 = n2.size(); 
  if (len1 == 0) return n2; 
  if (len2 == 0) return n1; 
  int carry = 0; 
  int a, b, sum; 
  int i = len1 - 1, j = len2 - 1; 
  while (i >= 0 || j >= 0 || carry > 0) { 
    a = 0; 
    b = 0; 
    if (i >= 0) a = n1[i] - '0'; 
    if (j >= 0) b = n2[j] - '0'; 
    sum = a + b + carry; 
    if (sum >= 10) carry = 1; 
    else carry = 0; 
    result.push_back(sum % 10 + '0'); 
    i--; 
    j--; 
  } 
  reverse