剑指 Offer II 025. 链表中的两数相加

[题目链接](剑指 Offer II 025. 链表中的两数相加)

先把相加的两个整数单链表进行反转，再相加。 反转链表可看[第24题](剑指 Offer II 024. 反转链表_WoAiChiXueGao_的博客-CSDN博客)。再将相加的结果进行反转相加的过程中注意进位什么的。 代码

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode last = null;
        ListNode node = head;
        while(node != null) {
            ListNode next = node.next;
            node.next =last;
            last = node;
            node = next;
        }
        //不返回node的原因时，此时的node为null
        return last;
    }
    public ListNode add(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);;
        ListNode sumNode = dummy;
        int carry = 0;
        while(l1 != null || l2 != null) {
            int num1 = l1 == null ? 0 : l1.val;
            int num2 = l2 == null ? 0 : l2.val;
            int sum = num1 + num2 + carry;
            carry = sum >= 10 ? 1 : 0; // 看是否有进位
            sum = sum >= 10 ? sum-10 : sum;

            ListNode node = new ListNode(sum);
            sumNode.next = node;
            sumNode = sumNode.next;

            l1 = l1 == null ? null : l1.next;
            l2 = l2 == null ? null : l2.next;
        }
        if(carry > 0) {
            sumNode.next = new ListNode(carry);
        }
        return dummy.next;
    }
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        l1 = reverseList(l1);
        l2 = reverseList(l2);
        ListNode sumNode = add(l1, l2);
        ListNode ans = reverseList(sumNode);
        return ans;
     }
}