No.21 Merge Two Sorted List

No.21 Merge Two Sorted List

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.

很简单，不多说，主要在于注意细节

/**
 * 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 *head, *current;
        //使用头结点的方法：ListNode helper = new ListNode(0); helper.next = head1;
        //他们都用的是将l2的节点插入l1中的方法

        if(l1->val <= l2->val)
        {    
            head = l1;
            l1 = l1->next;
        }
        else
        {
            head = l2;
            l2 = l2->next;
        }
        
        current = head;
        
        while(l1 && l2)
        {
            if(l1->val <= l2->val)
            {    
              current->next = l1;
              l1 = l1->next;
            }
            else
             {    
              current->next = l2;
              l2 = l2->next;
            }
            current = current->next;
        }
        
        if(l1)
            current->next = l1;
        if(l2)
            current->next = l2;
            
        return head;
    }
};