LeetCode力扣21.合并两个有序链表 Java

两种解法：递归和新建链表

新建一个链表：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head=new ListNode(); //头节点用于返回，不动
        ListNode temp=head;

        while (l1!=null&&l2!=null){
            if(l1.val<l2.val){
                temp.next=l1;
                l1=l1.next;
            }else {
                temp.next=l2;
                l2=l2.next;
            }
            temp=temp.next;
        }
        
        temp.next=l1==null?l2:l1;
        return head.next;
    }
}

递归：

class Solution{
	public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
		if(l1=null){
			return l2;
		}
		if(l2=null){
			return l1;
		}
		if(l1.val<l2.val){
			l1.next=mergeTwoLists(l1.next,l2);
			return l1;
		}else{
			l2.next=mergeTwoLists(l1,l2.next);
			return l2;
		}
	}
}