Addition of two numbers using Linked List

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */

struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2){
    struct ListNode* l3 =  (struct ListNode*)malloc(sizeof(struct ListNode));
    l3->val = 0;
    l3->next = NULL;
    struct ListNode* Result = l3;
    unsigned int sum = 0,carry = 0;
    while(l1 != NULL || l2 != NULL){
      if(l2==NULL){
          l2 = (struct ListNode*)malloc(sizeof(struct ListNode));
          l2->next = NULL;
          l2->val = 0;
      }
      if(l1==NULL){
          l1 = (struct ListNode*)malloc(sizeof(struct ListNode));
          l1->next = NULL;
          l1->val = 0;
      }
      sum = l1->val + l2->val;
      int temp = sum;
      if(sum/10 >= 1){
          sum = sum%10;
      }
      else{
          sum = l1->val + l2->val;
      }
      l3->next =  (struct ListNode*)malloc(sizeof(struct ListNode));
      if(sum == 9 && carry == 1){
          l3->next->val = 0;
          carry = 1;
      }
      else{
      l3->next->val = sum + carry;
      l3->next->next = NULL;
      
     if(temp/10 >= 1){
      carry = 1;   
     }
      else{
      carry = 0;
     }
    }
      l1= l1->next;
      l2 = l2->next;
      l3 = l3->next;
  }
    if(carry == 1){
      l3->next =  (struct ListNode*)malloc(sizeof(struct ListNode));
      l3->next->val = 1;
      l3->next->next = NULL;
    }
    Result = Result->next;
    return Result;
}