【leetcode链表】合并两个有序链表

将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

示例：

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

递归实现：
 

/**
 * 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;
        }
        if(l2==null) {
            return l1;
        }
        
        ListNode head = null;
        
        if(l1.val<l2.val) {
            head = l1;
            head.next = mergeTwoLists( l1.next, l2);
        }else{
            head = l2;
            head.next = mergeTwoLists( l1, l2.next);
        }
        
        return head;
        

    }
}

非递归实现：
 

    /**
     * 循环
     * @param n1
     * @param n2
     * @return
     */
    public static Node mergeTwoListByLoop(Node n1, Node n2){
        if(n1 == null){
            return n2;
        }
        if(n2 == null){
            return n1;
        }
 
        Node head, temp;
        if(n1.data < n2.data){
            temp = n1;
            n1 = n1.next;
        }else{
            temp = n2;
            n2 = n2.next;
        }
        head = temp;
 
        while(n1 != null && n2 != null){
            if(n1.data < n2.data){
                temp.next = n1;
                n1 = n1.next;
            }else{
                temp.next = n2;
                n2 = n2.next;
            }
            temp = temp.next;
        }
 
        if(n1 == null){
            temp.next = n2;
        }
        if(n2 == null){
            temp.next = n1;
        }
 
        return head;
    }