C++ 代码

/**
 * 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) return l2;
        if(!l2) return l1;
        ListNode* dummy = new ListNode(-1);
        auto cur = dummy;
        while(l1 && l2){
            if(l1->val <= l2->val) cur->next = l1, l1 = l1->next;
            else cur->next = l2, l2 = l2->next;
            cur = cur->next;
        }
        if(l1) cur->next = l1;
        if(l2) cur->next = l2;
        return dummy->next;
    }  
};