（链表）11-链表相加

 

 

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     *
     * @param head1 ListNode类
     * @param head2 ListNode类
     * @return ListNode类
     */
    public ListNode addInList (ListNode head1, ListNode head2) {
        // 翻转两个入参链表
        head1 = reverseList(head1);
        head2 = reverseList(head2);
        // 创建新链表作为返回结果
        ListNode root = new ListNode(-1);
        ListNode current = root;
        // 进位符号
        int flag = 0;
        // 依次相加
        while(head1 != null || head2 != null || flag != 0) {
            // 计算当前位的结果
            int val1 = (head1 == null) ? 0 : head1.val;
            int val2 = (head2 == null) ? 0 : head2.val;
            int temp = val1 + val2 + flag;
            flag = temp / 10;
            temp = temp % 10;
            // 返回结果中增加新的节点
            ListNode newNode = new ListNode(temp);
            current.next = newNode;
            current = current.next;
            // 移动到下一位进行计算
            if (head1 != null) {
                head1 = head1.next;
            }
            if (head2 != null) {
                head2 = head2.next;
            }
        }
        // 返回结果
        return reverseList(root.next);
    }

    public ListNode reverseList(ListNode head) {
        // 申请临时变量
        ListNode current = head;
        ListNode pre = null;
        // 依次调换指针的方向
        while (current != null) {
            // 记录下个要处理的节点
            ListNode next = current.next;
            // 调换当前节点的指针方向
            current.next = pre;
            // 移动到下个要处理的节点
            pre = current;
            current = next;
        }
        // 返回调换后的头节点
        return pre;
    }
}