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
     */
    // 反转链表
    ListNode reverse(ListNode head) {
        if (head == null)
            return head;
        ListNode cur = head;
        ListNode node = null;
        while (cur != null) {
            ListNode tail = cur.next;
            cur.next = node;
            node = cur;
            cur = tail;
        }
        return node;
    }
    public ListNode addInList (ListNode l1, ListNode l2) {
        l1 = reverse(l1);
        l2 = reverse(l2);
        ListNode head = null, tail = null;
        int carry = 0; //进位置 carry
        while (l1 != null || l2 != null) {
            int n1 = l1 != null ? l1.val : 0;
            int n2 = l2 != null ? l2.val : 0;
            int sum = n1 + n2 + carry;
            if (head == null) {
                head = tail = new ListNode(sum % 10);
            } else {
                tail.next = new ListNode(sum % 10);
                tail = tail.next;
            }

            carry = sum / 10;
            if (l1 != null) {
                l1 = l1.next;
            }
            if (l2 != null) {
                l2 = l2.next;
            }
        }

        if (carry > 0) {
            tail.next = new ListNode(carry);
        }
        head = reverse(head);
        return head;
    }
}

这道题和LeetCode第二道题其实是一样的，把链表反转相加时，从最低位到最高位逐位处理，并将结果按顺序添加到结果链表中。

进位值为 carry，则它们的和为 n1+n2+carry。

class ListNodeSolution {

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

    public static ListNode createList(int[] values) {
        ListNode head = new ListNode(values[0]);
        ListNode current = head;
        for (int i = 1; i < values.length; i++) {
            current.next = new ListNode(values[i]);
            current = current.next;
        }
        return head;
    }

    public static void printList(ListNode node) {
        while (node != null) {
            System.out.print(node.val);
            if (node.next != null) {
                System.out.print("->");
            }
            node = node.next;
        }
        System.out.println();
    }

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode head = null, tail = null;
        int carry = 0;
        while(l1!=null || l2!=null){
            int n1 = l1!=null ? l1.val : 0;
            int n2 = l2 !=null ? l2.val : 0;
            int sum = n1+n2 + carry;
            if(head == null){
                head = tail = new ListNode(sum % 10);
            }else{
                tail.next = new ListNode(sum % 10);
                tail = tail.next;
            }

            carry = sum / 10;
            if(l1!=null){
                l1 = l1.next;
            }
            if(l2!=null){
                l2 = l2.next;
            }
        }

        if(carry > 0){
            tail.next = new ListNode(carry);
        }
        return head;
    }

    public static void main(String[] args) {
        ListNode l1 = createList(new int[]{2, 4, 3});
        ListNode l2 = createList(new int[]{5, 6, 4});
        printList(l1);
        printList(l2);
        ListNodeSolution solution = new ListNodeSolution();
        ListNode listNode = solution.addTwoNumbers(l1, l2);
        printList(listNode);
    }
}