题目1: 两数相加

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode pre = new ListNode(0);
        ListNode cur = pre;
        int c = 0;//表示进位
        while(l1 != null || l2 != null){
            int x = l1 == null ? 0 : l1.val;
            int y = l2 == null ? 0 : l2.val;
            int sum = x + y + c;
            c = sum / 10;
            sum = sum % 10;
            cur.next = new ListNode(sum);
            cur = cur.next;
            if(l1 != null){
                l1 = l1.next;
            }
            if(l2 != null){
                l2 = l2.next;
            }
        }
        if(c == 1){
            cur.next = new ListNode(c);
        }
        return pre.next;
    }
}