see-21. Merge Two Sorted Lists

合并二个有序链表：

逐步遍历链表A,B，同时比较A,B对应位置的属性值大小，将属性值较小的节点加入新链表，直到链表A,B遍历完毕；

/**
 * 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) 
    {
        ListNode *res=(ListNode*)malloc(sizeof(ListNode));
        res->next=NULL;
        ListNode *ans=res;
       
        while(l1!=NULL&&l2!=NULL)
        {
           
            if(l1->val<l2->val)
            {
                ans->next=l1;
                ans=ans->next; //add new node into the res
                
                l1=l1->next;//l1 the next node
            }
            else
            {
                ans->next=l2;
                ans=ans->next; // add new node into the res
                
                l2=l2->next;//l2 the next node
            }
        }
        
        if(l1==NULL)
            ans->next=l2;
        else
            ans->next=l1;  //add the rest nodes into the res
        
        return res->next;
    }
};