addTwoHugeNumbers

addTwoHugeNumbers.js

// Singly-linked lists are already defined with this interface:
// function ListNode(x) {
//   this.value = x;
//   this.next = null;
// }
//
function addTwoHugeNumbers(a, b) {
  var sum = null,
      tmp,
      carry = 0,
      x, y,
      a = revertList(a),
      b = revertList(b);
  
  while (a || b || carry) {
        x = a ? a.value : 0;
        y = b ? b.value : 0;
        
        tmp = new ListNode((carry + x + y) % 1e4);
        tmp.next = sum;
        sum = tmp;
        carry    = (carry + x + y) / 1e4 | 0;
        
        if (a) a = a.next;
        if (b) b = b.next;
    }
  
  return sum;
}


function revertList(head) {
  let node = head,
      previous,
      tmp;

  while (node) {
    // save next before we overwrite node.next!
    tmp = node.next;

    // reverse pointer
    node.next = previous;

    // step forward in the list
    previous = node;
    node = tmp;
  }

  return previous;
}

You're given 2 huge integers represented by linked lists. Each linked list element is a number from 0 to 9999 that represents a number with exactly 4 digits. The represented number might have leading zeros. Your task is to add up these huge integers and return the result in the same format.

Example

For a = [9876, 5432, 1999] and b = [1, 8001], the output should be
addTwoHugeNumbers(a, b) = [9876, 5434, 0].

Explanation: 987654321999 + 18001 = 987654340000.

For a = [123, 4, 5] and b = [100, 100, 100], the output should be
addTwoHugeNumbers(a, b) = [223, 104, 105].

Explanation: 12300040005 + 10001000100 = 22301040105.

https://app.codesignal.com/interview-practice/task/RvDFbsNC3Xn7pnQfH/description