21.merge-two-sorted-lists.cpp

/*
 * @lc app=leetcode id=21 lang=cpp
 *
 * [21] Merge Two Sorted Lists
 */

// @lc code=start
/**
 * 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) {
        if (list1 == NULL) { return list2;}
        if (list2 == NULL) {return list1;}

        if (list1-> val <= list2->val){
            list1-> next = mergeTwoLists(list1->next,list2);
            return list1;
        }else{
            list2->next = mergeTwoLists(list1,list2->next);
            return list2;
        }
    }
};
// @lc code=end