445. Add Two Numbers II

• You are given two non-empty linked lists representing two
  non-negative integers. The most significant digit comes first and
  each of their nodes contain a single digit. Add the two numbers and
  return it as a linked list.

  You may assume the two numbers do not contain any leading zero,
  except the number 0 itself.

  Follow up: What if you cannot modify the input lists? In other words,
  reversing the lists is not allowed.

  Example:

  Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 8 -> 0 -> 7

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     struct ListNode *next; * }; */int getListLength(const struct ListNode *l){    struct ListNode *pNode = l;    int count = 0;    while(pNode){        count++;        pNode = pNode->next;    }     return count;}int addTwoNumbers_1(struct ListNode *l1,int len1,struct ListNode *l2,int len2){    int carry = 0;    int sum = 0;    if(len1 == 0 || len2 == 0||       l1 == NULL || l2 == NULL){        return 0;    }    if(len1>len2){        carry = addTwoNumbers_1(l1->next,len1-1,l2,len2);        sum = carry + l1->val;        l1->val = sum%10;    }    if(len1 == len2){        carry = addTwoNumbers_1(l1->next,len1-1,l2->next,len2-1);        sum = carry + l1->val + l2->val;        l1->val = sum%10;    }       if(len1<len2){        carry = addTwoNumbers_1(l1,len1,l2->next,len2-1);        sum = carry + l2->val;        l2->val = sum%10;    }    carry = sum/10;    return carry;}struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {    int len1 = 0;    int len2 = 0;    int carry = 0;    struct ListNode *pNode = NULL;    struct ListNode *head = NULL;    len1 = getListLength(l1);    len2 = getListLength(l2);    if(len1>len2){        carry = addTwoNumbers_1(l1,len1,l2,len2);        pNode = l1;    }    if(len1<=len2){        carry = addTwoNumbers_1(l2,len2,l1,len1);        pNode = l2;     }    if(carry>0){        head = (struct ListNode *)malloc(sizeof(struct ListNode));        head->val = carry;        head->next = pNode;        pNode = head;    }    return pNode;}

/*java solution using stack*/public class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        Stack<Integer> s1 = new Stack<Integer>();        Stack<Integer> s2 = new Stack<Integer>();        while(l1 != null) {            s1.push(l1.val);            l1 = l1.next;        };        while(l2 != null) {            s2.push(l2.val);            l2 = l2.next;        }        int sum = 0;        ListNode list = new ListNode(0);        while (!s1.empty() || !s2.empty()) {            if (!s1.empty()) sum += s1.pop();            if (!s2.empty()) sum += s2.pop();            list.val = sum % 10;            ListNode head = new ListNode(sum / 10);            head.next = list;            list = head;            sum /= 10;        }        return list.val == 0 ? list.next : list;    }}