scala: leetcode solution for 2. add two numbers

scala/2-Add-Two-Numbers.scala

@@ -0,0 +1,42 @@
+/**
+ * Definition for singly-linked list.
+ * class ListNode(_x: Int = 0, _next: ListNode = null) {
+ *   var next: ListNode = _next
+ *   var x: Int = _x
+ * }
+ */
+object Solution {
+  def addTwoNumbers(l1: ListNode, l2: ListNode): ListNode = {
+    def nonNull(l: ListNode): Boolean = l != null
+    def next(l: ListNode): ListNode = if (nonNull(l)) l.next else null
+    def getX(l: ListNode): Int = if (nonNull(l)) l.x else 0
+    def sumX(l1: ListNode, l2: ListNode, carry: Int): (ListNode, Int) = {
+      val sum = getX(l1) + getX(l2) + carry
+      new ListNode(sum % 10) -> sum / 10
+    }
+
+    var carry = 0
+    val (sum, newCarry) = sumX(l1, l2, carry)
+    carry = newCarry
+
+    var nextDigit = sum
+    var next1 = next(l1)
+    var next2 = next(l2)
+
+    while (nonNull(next1) || nonNull(next2)) {
+      val (sum, newCarry) = sumX(next1, next2, carry)
+
+      nextDigit.next = sum
+      nextDigit = nextDigit.next
+
+      carry = newCarry
+      next1 = next(next1)
+      next2 = next(next2)
+    }
+
+    if (carry > 0)
+      nextDigit.next = new ListNode(carry)
+
+    sum
+  }
+}