【算法】合并两个有序链表(easy)——递归算法

/**
 * 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) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) 
    {
        //递归方法
        ListNode* head = dfs(list1,list2);
        return head;
    }
    ListNode* dfs(ListNode* list1, ListNode* list2)
    {
        if(list1 == nullptr) return list2;
        if(list2 == nullptr) return list1;
        if(list1->val < list2->val) {list1->next = dfs(list1 -> next,list2);return list1;}
        else {list2->next = dfs(list1, list2->next);return list2;}
    }
};