LeetCode-021合并两个有序链表

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {

    if(l1==null){//l1空直接返回l2
        return l2;
    }
    if(l2==null){//l2空直接返回l1
        return l1;
    }
    ListNode res = new ListNode();
    ListNode head = res;//保留第一个结点

    while(l1!=null&&l2!=null){//l1、l2都不为空，进行比较，head创建小的结点
        if(l1.val>l2.val){
            head.next = new ListNode(l2.val);
            l2 = l2.next;
        }else{
            head.next = new ListNode(l1.val);
            l1 = l1.next;
        }
        head=head.next;
    }
    if(l1==null){//剩余的l2加入head
        while(l2!=null){
            head.next = new ListNode(l2.val);
            l2 = l2.next;
            head=head.next;
        }
    }
    if(l2==null){//剩余的l1加入head
        while(l1!=null){
            head.next = new ListNode(l1.val);
            l1 = l1.next;
            head=head.next;
        }
    }
    return res.next;
}