21. Merge Two Sorted Lists

• Recursion

/**
 * 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) return list2;
        if (!list2) return list1;
        if (list1->val < list2->val)
        {
            list1->next = mergeTwoLists(list1->next, list2);
            return list1;
        }
        else
        {
            list2->next = mergeTwoLists(list2->next, list1);
            return list2;
        }
    }
};

• T: $O(n + m)$

• S: $O(n + m)$

• Iteration

/**
 * 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)
    {
        ListNode* dummy = new ListNode(-1);
        ListNode* cur = dummy;

        while (list1 && list2)
        {
            if (list1->val < list2->val)
            {
                cur->next = list1;

                list1 = list1->next;
            }
            else
            {
                cur->next = list2;
                list2 = list2->next;
            }
            cur = cur->next;
        }
        if (!list1) cur->next = list2;
        if (!list2) cur->next = list1;
        return dummy->next;
    }
};

• T: $O(n + m)$
• S: $O(1)$