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.

Solution:

Merge sort 的核心部分。（or Merge two array）

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution 
{
    public ListNode mergeTwoLists(ListNode l1, ListNode l2)
    {
        ListNode result = new ListNode(0);
        ListNode head = result;
        while (l1 != null || l2 != null)
        {
            if (l1 == null)
            {
                result.next = l2;
                l2 = l2.next;
                result = result.next;
            }
            else if (l2 == null)
            {
                result.next = l1;
                l1 = l1.next;
                result = result.next;
            }
            else
            {
                if (l1.val > l2.val)
                {
                    result.next = l2;
                    l2 = l2.next;
                    result = result.next;
                } else
                {
                    result.next = l1;
                    l1 = l1.next;
                    result = result.next;
                }
            }
        }
        return head.next;
    }
}