leetcode 30: Merge Two Sorted Lists

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.

/**
 * 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
        
        if(!l2) return l1;
        if(!l1) return l2;
        
        ListNode* head = NULL;
        ListNode** pre = &head;
        
        while( l1 && l2){
            
            if(l1->val < l2->val){
                *pre = l1;
                l1 = l1->next;
            } else {
                *pre = l2;
                l2 = l2->next;
            }
            
            pre = &((*pre)->next);  
        }
        
        if(l1) *pre = l1;
        if(l2) *pre = l2;
        
        return head;
        
    }
};