Add Two Numbers

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

递归解法

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    return add(l1, l2, 0);
}

private ListNode add(ListNode a, ListNode b, int carry) {
    if (a == null && b == null) {
        if (carry == 0) {
            return null;
        }
        return new ListNode(carry);
    }

    int value = carry;
    if (a != null) {
        value += a.val;
    }
    if (b != null) {
        value += b.val;
    }
    ListNode re = new ListNode(value % 10);
    re.next = add(a == null ? null : a.next, b == null ? null : b.next, value/10);
    return re;
}

非递归解法

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode dummy = new ListNode(-1);
    ListNode cur = dummy;
    int carry = 0;
    while (l1 != null || l2 != null) {
        int value = (l1 != null ? l1.value : 0) + (l2 != null ? l2.value : 0) + carry;
        carry = value / 10;
        cur.next = new ListNode(value % 10);

        if (l1 != null) {
            l1 = l1.next;
        }
        if (l2 != null) {
            l2 = l2.next;
        }
        cur = cur.next;
    }
    if (carry > 0) {
        cur.next = new ListNode(carry);
    }
    return dummy.next;
}