两个链表的归并

/**
 * 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
        ListNode *p = l1, *q = l2, dummy(-1);
        ListNode *curr = &dummy;
        while (p || q) {
            int a = INT_MAX, b = INT_MAX;
            if (p) {
                a = p->val;
            }
            if (q) {
                b = q->val;
            }
            if (a < b) {
                curr->next = p;
                p = p->next;
            }
            else {
                curr->next = q;
                q = q->next;
            }
            curr = curr->next;
        }
        return dummy.next;
    }
};