力扣--两数之和

类似于数组的高精度加法，但是需要知道连接链表

优化前：

/**
 * 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 ans = new ListNode();
  let head = null;
  let t = 0;
  while (l1 && l2) {
    let value = (l1.val + l2.val + t) % 10;
    if (!head) {
      ans.val = value;
      head = ans;
    } else {
      let node = new ListNode();
      node.val = value;
      head.next = node;
      head = node;
    }

    t = parseInt((l1.val + l2.val + t) / 10);
    l1 = l1.next;
    l2 = l2.next;
  }
  while (l1 !== null) {
    let node = new ListNode();
    node.val = (l1.val + t) % 10;
    head.next = node;
    head = node;
    t = parseInt((l1.val + t) / 10);

    l1 = l1.next;
  }
  while (l2 !== null) {
    let node = new ListNode();
    node.val = (l2.val + t) % 10;
    head.next = node;
    head = node;
    t = parseInt((l2.val + t) / 10);

    l2 = l2.next;
  }
  if (t) {
    let node = new ListNode();
    node.val = 1;
    head.next = node;
  }
  return ans;
};

其实可以写在一个while循环里面，看题解优化后：还比上面的慢了1ms，但是代码风格更好

/**
 * 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 head = null;
  let tail = null;
  let t = 0;

  while (l1 || l2) {
    const a = l1 ? l1.val : 0;
    const b = l2 ? l2.val : 0;
    let value = (a + b + t) % 10;
    let node = new ListNode();
    if (!head) {
      head = tail = node;
      head.val = value;
    } else {
      node.val = value;
      tail.next = node;
      tail = node;
    }
    t = parseInt((a + b + t) / 10);
    if (l1) {
      l1 = l1.next;
    }
    if (l2) {
      l2 = l2.next;
    }
  }
  if (t) {
    let node = new ListNode(t);
    tail.next = node;
  }
  return head;
};