【LeetCode】21. Merge Two Sorted 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==NULL){
            return l2;
        }
        if(l2==NULL){
            return l1;
        }
        ListNode* p;
        ListNode* head;
        ListNode* p1=l1;
        ListNode* p2=l2;
        if(p1->val<p2->val){
            p=p1;
            p1=p1->next;
        }else{
            p=p2;
            p2=p2->next;
        }
        head=p;
        while(p1!=NULL&&p2!=NULL)
        {
             if(p1->val<p2->val){
                p->next=p1;
                p1=p1->next;
                p=p->next;
            }else{
                p->next=p2;
                p2=p2->next;
                p=p->next;
            }
        }if(p1!=NULL){
            p->next=p1;
        }else{
            p->next=p2;
        }
        return head;
    }
};