Update addTwoNumbers.md

addTwoNumbers.md

@@ -30,6 +30,45 @@ Explanation: 342 + 465 = 807.
  * @return {ListNode}
  */
 var addTwoNumbers = function(l1, l2) {
+    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;
 };
 ```