[2] Add two numbers

algorithm/javascript/leetcode/2-add-two-numbers.js

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+// https://leetcode.com/problems/add-two-numbers/
+
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+/**
+ * @param {ListNode} l1
+ * @param {ListNode} l2
+ * @return {ListNode}
+ */
+var addTwoNumbers = function(l1, l2) {
+  let node = null;
+  const carry = arguments[2];
+  if (l1 || l2) {
+    const val1 = l1 ? l1.val : 0;
+    const val2 = l2 ? l2.val : 0;
+    const next1 = l1 ? l1.next : null;
+    const next2 = l2 ? l2.next : null;
+    const val = carry ? val1 + val2 + 1 : val1 + val2;
+    node = new ListNode(val % 10);
+    node.next = addTwoNumbers(next1, next2, val >= 10);
+  } else if (carry) {
+    node = new ListNode(1);
+    node.next = null;
+  }
+  return node;
+};