LeetCode OJ：Merge Two Sorted Lists（合并两个链表）

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

注意题目要求合并的时候不能新建节点，直接使用原来的节点，比较简单，代码如下：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(l1 == NULL) return l2;
        if(l2 == NULL) return l1;
        ListNode * root = new ListNode(-1);
        ListNode * helper = root;
        while(l1!=NULL && l2!=NULL){
            if(l1->val <= l2->val)
                root->next = l1, l1=l1->next;
            else if(l1->val > l2->val)
                root->next = l2, l2=l2->next;
            root = root->next;
        }
        while(l1!=NULL){
            root->next = l1;
            l1 = l1->next;
            root = root->next; 
        }
        while(l2!=NULL){
            root->next = l2;
            l2 = l2->next;
            root = root->next; 
        }
        return helper->next;
    }
};

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode helper = new ListNode(0);
        ListNode ret = helper;
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        while(l1 != null && l2 != null){
            if(l1.val < l2.val){
                helper.next = l1;
                l1 = l1.next;
            }else{
                helper.next = l2;
                l2 = l2.next;
            }
            helper = helper.next;
        }
        while(l1 != null){
            helper.next = l1;
            l1 = l1.next;
            helper = helper.next;
        }
        while(l2 != null){
            helper.next = l2;
            l2 = l2.next;
            helper = helper.next;
        }
        return ret.next;
    }
}