445. 两数相加 II

reverse

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode r1 = reverse(l1);
        ListNode r2 = reverse(l2);
        int carry = 0;
        ListNode dummy = new ListNode();
        ListNode cur = dummy;
        while (r1 != null || r2 != null || carry != 0) {
            int num1 = r1 == null ? 0 : r1.val;
            int num2 = r2 == null ? 0 : r2.val;
            int sum = num1 + num2 + carry;
            int curSum = sum % 10;
            carry = sum / 10;
            cur.next = new ListNode(curSum);//cur = new ListNode(curSum);
            cur = cur.next;
            r1 = r1 == null ? null : r1.next;
            r2 = r2 == null ? null : r2.next;
        }
        return reverse(dummy.next);
        // return reverse(dummy.next);
    }

    public ListNode reverse(ListNode node) {
        ListNode pre = null, cur = node;
        while (cur != null) {
            ListNode tmp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = tmp;
        }
        return pre;
    }
}

stack

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> s1 = new Stack<>();
        Stack<Integer> s2 = new Stack<>();
        while (l1 != null) {
            s1.push(l1.val);
            l1 = l1.next;
        }
        while (l2 != null) {
            s2.push(l2.val);
            l2 = l2.next;
        }

        int carry = 0;
        ListNode pre = null, cur = null;
        while (!s1.isEmpty() || !s2.isEmpty() || carry != 0) {
            int num1 = s1.isEmpty() ? 0 : s1.pop();
            int num2 = s2.isEmpty() ? 0 : s2.pop();
            int sum = num1 + num2 + carry;
            int curSum = sum % 10;
            carry = sum / 10;
            cur = new ListNode(curSum);
            cur.next = pre;
            pre = cur;
        }
        return cur;
    }
}