leetcode 2. Add Two Numbers

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {        int len1 = 0, len2 = 0;        for(ListNode* p = l1; p; len1++, p = p->next);        for(ListNode* p = l2; p; len2++, p = p->next);        if(len1 > len2) {        ListNode *t = l2;        while(t->next) t = t->next;        for(int i = len2; i < len1; i++) {        ListNode *p = new ListNode(0);        t->next = p;        t = p;        }        }        else {        ListNode *t = l1;        while(t->next) t = t->next;        for(int i = len1; i < len2; i++) {        ListNode *p = new ListNode(0);        t->next = p;        t = p;        }        }        ListNode *p = new ListNode(l1->val + l2->val), *head;        head = p;for(l1 = l1->next, l2 = l2->next; l1 && l2; l1 = l1->next, l2 = l2->next) {p->next = new ListNode(l1->val + l2->val);p = p->next;}for(p = head; p; p = p->next)if(p->val >= 10) {p->val %= 10;if(p->next) p->next->val++;else p->next = new ListNode(1);}return head;    }};