21. Merge Two Sorted Lists

插入表二的值到表一中去以后，要将比较的值也变成这个插入的值

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode pos1 = l1;
        ListNode pos2 = l2;
        ListNode dummy = new ListNode (0);
        dummy.next=l1;
        ListNode pre = dummy;
        while(pos1!=null&&pos2!=null)
        {
            while(pos1!=null&&pos1.val<=pos2.val)
            {
                pos1=pos1.next;
                pre=pre.next;
            }
            if(pos1==null)break;
            // first pos1.val>pos2.val insert pos2 before pos1 ;
           ListNode temp = pos2.next;
           pre.next=pos2;
           pos2.next=pos1;
            // now pos1 is the inset val ,so change pos1 
           pos2=temp;
           pos1=pre.next;
        }
        if(pos1==null)
        {
            pre.next=pos2;
        }
        return dummy.next;
    }
}