每3日一篇leetcode，第二题：AddTwoNumbers

1.两个非空链表，分别倒叙保存数字，两个链表相加并且返回一个链表

    //2ms 45.5m
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

        LinkedList<Integer> linkedList1 = new LinkedList<Integer>();
        LinkedList<Integer> linkedList2 = new LinkedList<Integer>();
        while (l1 != null || l2 != null) {
            linkedList1.add(0, l1 == null ? 0 : l1.val);
            l1 = l1 != null ? l1.next : null;

            linkedList2.add(0, l2 == null ? 0 : l2.val);
            l2 = l2 != null ? l2.next : null;
        }
        return getListNode(linkedList1, linkedList2, false);
    }

    private ListNode getListNode(LinkedList<Integer> ll1, LinkedList<Integer> ll2, boolean isAdd1) {
        int num1 = ll1.getLast();
        int num2 = ll2.getLast();
        int left = num1 + num2 + (isAdd1 ? 1 : 0);
        if (left >= 10) {
            isAdd1 = true;
            left = left % 10;
        } else {
            isAdd1 = false;
        }
        ListNode listNode = new ListNode(left);
        ll1.removeLast();
        ll2.removeLast();
        if (!ll1.isEmpty() && !ll2.isEmpty()) {
            listNode.next = getListNode(ll1, ll2, isAdd1);
        } else if (isAdd1) {
            listNode.next = new ListNode(1);
        }
        return listNode;
    }

    //2 ms	46 MB
    public ListNode addTwoNumbers2(ListNode l1, ListNode l2) {
        ListNode dummyHead = new ListNode(0);
        ListNode p = l1, q = l2, curr = dummyHead;
        int carry = 0;
        while (p != null || q != null) {
            int x = (p != null) ? p.val : 0;
            int y = (q != null) ? q.val : 0;
            int sum = carry + x + y;
            carry = sum / 10;
            curr.next = new ListNode(sum % 10);
            curr = curr.next;
            if (p != null) p = p.next;
            if (q != null) q = q.next;
        }
        if (carry > 0) {
            curr.next = new ListNode(carry);
        }
        return dummyHead.next;
    }


    public class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
        }

    }