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// 大数相加

function add(a, b) {
  var carry = 0;
  var result = [];
  var sum = 0;
  var minLength = Math.min(a.length, b.length);
  var maxLength = Math.max(a.length, b.length);
  var aArrRe = a.split("").reverse();
  var bArrRe = b.split("").reverse();
  var maxArrRe = aArrRe.length > bArrRe.length ? aArrRe : bArrRe;

  for (var index = 0; index < maxLength; index++) {
    if (index < minLength) {
      sum = parseInt(aArrRe[index]) + parseInt(bArrRe[index]) + carry;
    } else {
      sum = parseInt(maxArrRe[index]) + carry;
    }
    carry = parseInt(sum / 10);
    result.unshift(sum % 10);
  }
  carry && result.unshift(carry);
  return result.join("");
}
console.log(add("9999999", "1"));