import commonclasses.ListNode

/**
 * You are given the heads of two sorted linked lists list1 and list2.
 *
 * Merge the two lists in a one sorted list.
 * The list should be made by splicing together the nodes of the first two lists.
 *
 * Return the head of the merged linked list.
 *
 *
 *
 * Example 1:
 *
 *
 * Input: list1 = [1,2,4], list2 = [1,3,4]
 * Output: [1,1,2,3,4,4]
 * Example 2:
 *
 * Input: list1 = [], list2 = []
 * Output: []
 * Example 3:
 *
 * Input: list1 = [], list2 = [0]
 * Output: [0]
 *
 *
 * Constraints:
 *
 * The number of nodes in both lists is in the range [0, 50].
 * -100 <= Node.val <= 100
 * Both list1 and list2 are sorted in non-decreasing order.
 * @see <a href="https://leetcode.com/problems/merge-two-sorted-lists/">LeetCode</a>
 *
 * Example:
 * var li = ListNode(5)
 * var v = li.`val`
 * Definition for singly-linked list.
 * class ListNode(var `val`: Int) {
 *     var next: ListNode? = null
 * }
 */
fun mergeTwoLists(list1: ListNode?, list2: ListNode?): ListNode? {
    if (list1 == null) return list2
    if (list2 == null) return list1
    return if (list1.`val` < list2.`val`) {
        list1.next = mergeTwoLists(list1.next, list2)
        list1
    } else {
        list2.next = mergeTwoLists(list1, list2.next)
        list2
    }
}
