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.

/**
 * 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) {
        if(!l1)return l2;
        if(!l2)return l1;
        
        ListNode * p = l1;
        ListNode * q = l2;
        ListNode * res = new ListNode(-1);
        ListNode * tmp = res;
        while(p&&q){
            if(p->val <= q->val){
                tmp->next = p;
                tmp = p;
                p = p->next;
            }else if(p->val > q->val){
                tmp->next = q;
                tmp = q;
                q = q->next;
            }
            
        }
        
        if(p) tmp->next = p;
        if(q) tmp->next = q;

        return res->next;
    }
};