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# 思路

使用遞迴

# 參考程式碼

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
static auto fast_io = []
{
    ios::sync_with_stdio(false);
	cout.tie(nullptr);
	cin.tie(nullptr);
	return 0;
}();
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2)
    {
        if (l1 == NULL) return l2;
        if (l2 == NULL) return l1;
        if (l1->val < l2->val)
        {
            l1->next = mergeTwoLists(l1->next, l2);
            return l1;
        }
        else
        {
            l2->next = mergeTwoLists(l1, l2->next);
            return l2;
        }
    }
};