leetcode-2 两数相加

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function (l1, l2) {
  let cur = l1,
    flag = 0,
    prev = null;
  while (cur && l2) {
    const sum = cur.val + l2.val + flag;
    flag = sum >= 10 ? 1 : 0;
    cur.val = sum % 10;
    prev = cur;
    cur = cur.next;
    l2 = l2.next;
  }
  if (l2) {
    // l1 为空
    prev.next = l2;
    while(l2) {
      const sum = l2.val + flag;
      flag = sum >= 10 ? 1 : 0;
      l2.val = sum % 10;
      if (flag === 0) break;
      prev = l2;
      l2 = l2.next;
    }
  }
  if (cur) {
  // l2为空
    while(cur) {
      const sum = cur.val + flag;
      flag = sum >= 10 ? 1 : 0;
      cur.val = sum % 10;
      if (flag === 0) break;
      prev = cur;
      cur = cur.next;
    }
  }
  if (flag) {
    prev.next = new ListNode(flag);
  }
  return l1;
};