2、Add Two Numbers

Example
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.
难点
1、ListNode的操作。
2、考虑进位的问题。
3、这道题由于数字反序，所以实际上从首位开始相加正好符合笔算时候的顺序。
解法

public class ListNode {
    int val;
    ListNode next;
    ListNode(int x){
        this.val = x;
    }
}
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode p = l1;
        ListNode q = l2;
        ListNode head = new ListNode(0);
        ListNode curr = head;
        int sum =0;
        
        while(p != null || q != null){
            sum = sum / 10;
            if(p != null){
                sum += p.val;
                p = p.next;
            }
            if(q != null){
                sum += q.val;
                q = q.next;
            }
            curr.next = new ListNode(sum % 10);
            curr = curr.next;
        }
        if(sum >= 10){
            curr.next = new ListNode(1);
        }
        
        return head.next;
    }
}