LeetCode 2 - Add Two Numbers ( JAVA )

Difficulity : 

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

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

/**
 * 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) {
      ListNode prev = new ListNode(0);
      ListNode head = prev;
      int carry = 0;
       while (l1 != null || l2 != null || carry != 0) {
          ListNode cur = new ListNode(0);
          int sum = ((l2 == null) ? 0 : l2.val) + ((l1 == null) ? 0 : l1.val) + carry;
          cur.val = sum % 10;
          carry = sum / 10;
          prev.next = cur;  //使下以个指针指向cur
          prev = prev.next;   //移动一个指针
          l1 = (l1 == null) ? l1 : l1.next;
          l2 = (l2 == null) ? l2 : l2.next;
       }
    return head.next;
    }
};

总结：效率比较高，对本题的理解可以画内存图，head和prev同时指向一个内存空间，之后移动prev指针指向下一个。head链表在不断的加长。