题解 | 链表相加(二)

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head1 ListNode类
     * @param head2 ListNode类
     * @return ListNode类
     */
    public ListNode addInList (ListNode head1, ListNode head2) {
        int carry = 0, cur = 0;
        ListNode newHead = null;
        head1 = reverse(head1);
        head2 = reverse(head2);

        while (head1 != null || head2 != null) {
            int temp;
            if (head1 == null) {
                temp = head2.val + carry;
                head2 = head2.next;
            } else if (head2 == null) {
                temp = head1.val + carry;
                head1 = head1.next;
            } else {
                temp = head1.val + head2.val + carry;
                head1 = head1.next;
                head2 = head2.next;
            }
            carry = temp / 10;
            cur = temp % 10;
            if (newHead == null) {
                newHead = new ListNode(cur);
            } else {
                ListNode newNode = new ListNode(cur) ;
                newNode.next = newHead;
                newHead = newNode;
            }
        }
        if (carry != 0) { // 最后的进位直接加入即可
            ListNode newNode = new ListNode(carry) ;
            newNode.next = newHead;
            newHead = newNode;
        }
        return newHead;
    }

    public ListNode reverse(ListNode head) {
        ListNode prev = null;
        while (head != null) {
            ListNode next = head.next;
            head.next = prev;

            prev = head;
            head = next;
        }
        return prev;
    }
}