LeetCode 21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

public class MergeTwoSortedListsSolution {
    /**
     * 解法一
     */
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {

        //常規解法：利用迴圈
        ListNode result = null;//頭指標
        ListNode now = result;//尾指標

        while (l1 != null && l2 != null) {
            ListNode temp = null;
            if (l1.val > l2.val) {
                temp = l2;
                l2 = l2.next;
            } else {
                temp = l1;
                l1 = l1.next;
            }
            temp.next = null;
            if (result != null) {
                now.next = temp;
                now = now.next;

            } else {
                result = temp;
                now = result;
            }

        }

        if (now != null) {
            if (l1 != null) {
                now.next = l1;
            }
            if (l2 != null) {
                now.next = l2;
            }
        } else {
            if (l1 != null) {
                return l1;
            }
            if (l2 != null) {
                return l2;
            }
        }

        return result;
    }

    /**
     * 解法二
     */
    public ListNode merge(ListNode l1, ListNode l2) {

        /**
         * 遞迴解法
         * 遞迴邊界：l1==null,l2==null
         * 每遞迴一次問題向下求解一次
         *
         */
        if (l1 == null) return l2;
        if (l2 == null) return l1;

        if (l1.val < l2.val) {
            l1.next = merge(l1.next, l2);
            return l1;
        } else {
            l2.next = merge(l1, l2.next);
            return l2;
        }
    }
}