Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null||l2==null)
        return l1==null? l2:l1;
        ListNode result = new ListNode(0);
        ListNode prev = result;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                prev.next = l1;
                l1 = l1.next;
            } else {
                prev.next = l2;
                l2 = l2.next;
            }
            prev = prev.next;
        }
        if (l1 != null) {
            prev.next = l1;
        }
        if (l2 != null) {
            prev.next = l2;
        }
        return result.next;
    }
}

Javascript

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function(l1, l2) {
    if(l1===null||l2===null) return l1? l1:l2;
    var tmp={val:0,next:null};
    var head=tmp;
    while(l1!==null&&l2!==null)
    {
        if(l1.val>=l2.val)
        {
            tmp.next=l2;
            l2=l2.next;
        }
        else if(l1.val<l2.val)
        {
           tmp.next=l1;
           l1=l1.next; 
        }
        tmp=tmp.next;
    }
    if(l2===null) tmp.next=l1;
    if(l1===null) tmp.next=l2;
    return head.next;
};

递归解

 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;
        }
}