原题链接：https://oj.leetcode.com/problems/merge-two-sorted-lists/

这道题用指针的指针实现，做法会非常简单干净。直接上代码

/**
 * 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 *head = NULL;
        ListNode **pp = &head;
        ListNode *p1 = l1;
        ListNode *p2 = l2;
        
        while (p1 && p2) {
            if (p1->val < p2->val) {
                *pp = p1;
                p1 = p1->next;
            } else {
                *pp = p2;
                p2 = p2->next;
            }
            pp = &(*pp)->next;
        }
        
        while (p1) {
            *pp = p1;
            pp = &(*pp)->next;
            p1 = p1->next;
        }
        
        while (p2) {
            *pp = p2;
            pp = &(*pp)->next;
            p2 = p2->next;
        }
        
        return head;
    }
};

[Leetcode] 21 - Merge Two Sorted Lists