Sword Finger Offer Brush Questions--GZ16--Merge two sorted linked lists

method one:

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

    ListNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {
    
    
    public ListNode Merge(ListNode list1,ListNode list2) {
    
    
        //先定义新链表的头结点
        ListNode newListNode = new ListNode(-1);
        //定义1个临时变量用于辅助遍历
        ListNode temp = newListNode;
        //开始遍历，合并
        while(list1 != null && list2 != null){
    
    
            if(list1.val <= list2.val){
    
    
                temp.next = list1;
                temp = temp.next;//新链表temp后移
                list1 = list1.next;
            } else {
    
    
                temp.next = list2;
                temp = temp.next;
                list2 = list2.next;
            }
        }
        if(list1 == null){
    
    
            temp.next = list2;
        }
        if(list2 == null){
    
    
            temp.next = list1;
        }
        //返回newListNode.next才是新链表的头结点
        return newListNode.next;
    }
}

Method 2: Recursion

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