【面试题17】合并两个排序的链表

输入两个单调递增的链表，输出两个链表合成后的链表，当然我们需要合成后的链表满足单调不减规则。

递归实现：

非递归方式：

public class Solution {
    public ListNode Merge(ListNode list1,ListNode list2) {
        if (list1 ==null)
            return list2;
        if (list2 ==null)
            return list1;
        ListNode head = null;
        ListNode tmp= null;
        if (list1.val < list2.val)
            {
            head = list1;
                list1 = list1.next;
        }
        else
        {
            head = list2;
                list2 = list2.next;
        }
        tmp = head;
        while (list1 != null && list2 != null)
            {
            if (list1.val < list2.val)
            {
               
            tmp.next = list1;
                list1 = list1.next;
                tmp = tmp.next;
               
        }
        else
        {
           tmp.next = list2;
                list2 = list2.next;
            tmp = tmp.next;
        }
        }
       
         while (list1 != null || list2 != null)
            {
        if (list1 ==null && list2 != null)
            {
            tmp.next = list2;
                list2 = list2.next;
            tmp = tmp.next;
        }
       
         if (list2 ==null && list1 != null)
            {
            tmp.next = list1;
                list1 = list1.next;
            tmp = tmp.next;
        }
         }
        return head;
       
    }
}