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) {        ListNode* head ;        ListNode* p1 = l1 ;        ListNode* p2  = l2;        int length1 =0 , length2 =0 ;        //特殊情况处理        if(l1 == NULL){            return l2;        }        if(l2 == NULL){            return l1;        }        //记录长度        //p1=l1;       // p2=l2;        while(p1 != NULL){            p1 = p1->next;            length1++;        }        while(p2 !=NULL){            p2 = p2->next;            length2++;        }        //决定谁是head,并把指针位置修改        if(l1->val < l2->val){            head = l1;            p1 = l1->next;            p2 = l2;        }        else{            head = l2;            p2 = l2->next;            p1 = l1;        }        ListNode *temp = head;        while(p1 != NULL && p2 != NULL){            if(p1->val < p2->val){                temp->next = p1;                //一开始没有移动temp,导致只有两个首尾两个数。                temp = p1;                p1 = p1->next;            }            else{                temp->next = p2;                 //一开始没有移动temp,导致只有两个首尾两个数。                temp = p2;                p2 = p2->next;            }        }        if(p1 == NULL){            temp->next = p2;        }        else{            temp->next = p1;        }        return head;    }};