var carried = 0; // 用于进位

const head = new ListNode();

const noop = {

val: 0,

next: null

};

let currentL1 = l1;

let currentL2 = l2;

let currentNode = head; // 返回的链表的当前node

let newNode; // 声明在外面节省内存

let previousNode; // 记录前一个节点，便于删除最后一个节点

while (currentL1 || currentL2) {

newNode = new ListNode(0);

currentNode.val =

((currentL1 || noop).val + (currentL2 || noop).val + carried) % 10;

currentNode.next = newNode;

previousNode = currentNode;

currentNode = newNode;

if ((currentL1 || noop).val + (currentL2 || noop).val + carried >= 10) {

carried = 1;

} else {

carried = 0;

}

currentL1 = (currentL1 || noop).next;

currentL2 = (currentL2 || noop).next;

}

if (carried) {

// 还有位没进呢

previousNode.next = new ListNode(carried)

} else {

previousNode.next = null;

}

return head;