const addString = function(num1, num2) {
  // 补零对齐
  while (num1.length > num2.length) num2 = '0' + num2;
  while (num1.length < num2.length) num1 = '0' + num1;

  let result = '';
  // 进位
  let carry = 0;
  // 加法，从个位开始计算
  for (let i = num1.length - 1; i >= 0; i--) {
    const sum = +num1[i] + +num2[i] + carry;
    result = (sum % 10) + result;
    carry = sum > 9 ? 1 : 0;
  }

  return carry == 1 ? '1' + result : result;
};