[LeetCode] Add January 12.

solved/LeetCode/Challenges/2020/January/12.kt

@@ -0,0 +1,25 @@
+class Solution {
+    fun addTwoNumbers(A: ListNode?, B: ListNode?): ListNode? {
+      var l1 = A
+      var l2 = B
+      var head = ListNode(0)
+      var current = head
+      var carry = 0
+
+      fun canAdd() = l1 != null || l2 != null || carry != 0
+
+      while (canAdd()) {
+        val here = (l1?.`val` ?: 0) + (l2?.`val` ?: 0) + carry
+        current.`val` = here % 10
+        carry = here / 10
+        l1 = l1?.let { it.next }
+        l2 = l2?.let { it.next }
+        if (canAdd()) {
+          current.next = ListNode(0)
+          current = current.next
+        }
+      }
+
+      return head
+    }
+}