21 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.

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) {
        ListNode *p1 = l1, *p2 = l2, *dummy = new ListNode(0);
        ListNode *p = dummy;
        int i1, i2;
        while ( p1 != NULL or p2 != NULL ) {
            if ( p1 == NULL ) i1 = INT_MAX;
            else i1 = p1->val;
            if ( p2 == NULL ) i2 = INT_MAX;
            else i2 = p2->val;
            if ( i1 < i2 ) { p->next = p1; p1 = p1->next;}
            else { p->next = p2; p2 = p2->next;}
            p = p->next;
        }
        return dummy->next;
    }
};