[LeetCode] 055: Merge Two Sorted Lists

[Problem]
Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

[Solution]

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
        // at least one of them is NULL
        if(NULL == l1)return l2;
        if(NULL == l2)return l1;
        
        // both of them are not NULL
        ListNode *head = NULL;
        if(l1->val < l2->val){
            head = new ListNode(l1->val);
            l1 = l1->next;
        }
        else{
            head = new ListNode(l2->val);
            l2 = l2->next;
        }
        
        // merge
        ListNode *p = head;
        while(l1 != NULL || l2 != NULL){
            if(NULL == l1){
                p->next = new ListNode(l2->val);
                l2 = l2->next;
            }
            else if(NULL == l2){
                p->next = new ListNode(l1->val);
                l1 = l1->next;
            }
            else if(l1->val < l2->val){
                p->next = new ListNode(l1->val);
                l1 = l1->next;
            }
            else{
                p->next = new ListNode(l2->val);
                l2 = l2->next;
            }
            p = p->next;
        }
        return head;
    }
};

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