/**
 * 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) {
        if(l1 == null){
            return l2;
        }else if(l2 == null){
            return l1;
        }
        ListNode head = l1;
        if(l1.val > l2.val){
            head = l2;
            l2 = l2.next;
        }else{
            l1 = l1.next;
        }
        ListNode cur = head;
        while(l1!=null&&l2!=null){
            if(l1.val <= l2.val){
                cur.next = l1;
                l1 = l1.next;
            }else{
                cur.next = l2;
                l2 = l2.next;
            }
            cur = cur.next;
        }
        if(l1 != null){
            cur.next = l1;
        }else if(l2 != null){
            cur.next = l2;
        }
        return head;
    }
}

设置dum结点，dum.next指向头结点

/**
 * 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 dum = new ListNode(0),cur = dum;
        while(l1!=null&&l2!=null){
            if(l1.val<=l2.val){
                cur.next = l1;
                l1 = l1.next;
            }else{
                cur.next = l2;
                l2 = l2.next;
            }
            cur = cur.next;
        }
        cur.next = l1!=null?l1:l2;
        return dum.next;
    }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        if(headA == null||headB == null){return null;}
        ListNode l1 = headA,l2 = headB;
        while(l1 != l2){
            if(l1 == null){l1 = headB;}
            else{l1 = l1.next;}
            if(l2 == null){l2 = headA;}
            else{l2 = l2.next;}
        }
        return l1;
    }
}