剑指：合并两个排序的链表

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode Merge(ListNode list1,ListNode list2) {
        if (list1 == null) return list2;
        if (list2 == null) return list1;
        
        ListNode p = list1;
        ListNode q = list2;
        
        ListNode ph = new ListNode(-1);
        ListNode cur = ph;
        
        while(p!=null && q!=null){
            if(p.val<q.val){
                ListNode t = p.next;
                cur.next = p;
                p.next = null;
                p = t;
            } else {
                ListNode t = q.next;
                cur.next = q;
                q.next = null;
                q = t;
            }
            cur = cur.next;
        }
        cur.next = p == null ? q : p;  
        return ph.next;
    }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {

    public ListNode Merge(ListNode list1, ListNode list2) {
        if(list1==null)
            return list2;
        if(list2==null)
            return list1;
        ListNode res = null;
        if(list1.val<list2.val){
            res = list1;
            res.next = Merge(list1.next, list2);
        }else{
            res = list2;
            res.next = Merge(list1, list2.next);
        }
        return res;
    }
}