题意

给定两个有序链表，要求合并成一个有序链表

分析

采用归并排序中使用的双指针归并的方法

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2) {
        if(l1 ==NULL || l2 == NULL){
            return l1==NULL?l2:l1;
        }

        ListNode* dummy = new ListNode(-1);
        ListNode* p = dummy;

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

        if(l1)p->next = l1;
        if(l2)p->next = l2;
        return dummy->next;
    }
};