21-Merge-Two-Sorted-Lists.cpp

/*
    Given heads of 2 sorted linked lists, merge into 1 sorted list
    Ex. list1 = [1,2,4], list2 = [1,3,4] -> [1,1,2,3,4,4]

    Create curr pointer, iterate thru, choose next to be lower one

    Time: O(m + n)
    Space: O(1)
*/

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