21 - Merge Two Sorted Lists

#easy

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Example 1:

Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2:
Input: list1 = [], list2 = []
Output: []

Example 3:
Input: list1 = [], list2 = [0]
Output: [0]

Constraints:

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both list1 and list2 are sorted in non-decreasing order.

**
 * 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 && list2 == NULL ) return NULL;
        ListNode * head = NULL;
        if ( list1 == NULL ) return list2;
        if ( list2 == NULL ) return list1;
        if ( list1 -> val > list2 -> val ) {
            head = list2;
            list2 = list2 -> next;
        }
        else {
            head = list1;
            list1 = list1 -> next;
        }
        ListNode * cur = head; 
        while ( list1 != NULL && list2 != NULL ) {
            if ( list1 -> val > list2 -> val ) {
                ListNode * tmp = list2 -> next;
                cur -> next = list2;
                list2 = tmp;
            }
            else {
                ListNode * tmp = list1 -> next;
                cur -> next = list1;
                list1 = tmp;
            }

            cur = cur -> next;
        }

        while ( list1 != NULL ) {
            cur -> next = list1;
            break;
        }
        while ( list2 != NULL ) {
            cur -> next = list2;
            break;
        }
        return head;
    }
};