LeetCode: Add Two Numbers 解题报告

Add Two Numbers
You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

https://oj.leetcode.com/problems/add-two-numbers/

解答：

比较简单，直接用2个指针往后移动，并且将2个list的值相加，并且计算上carry值。注意，在while循环中加上carry == 1的判断，这样可以自动在链尾加上carry 的值。

package Algorithms.list;

import Algorithms.algorithm.others.ListNode;

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class AddTwoNumbers {
    public static void main(String[] str) {
        ListNode n1 = new ListNode(9);
        
        ListNode l1 = new ListNode(1);
        ListNode l2 = new ListNode(9);
        ListNode l3 = new ListNode(9);
        ListNode l4 = new ListNode(9);
        ListNode l5 = new ListNode(9);
        ListNode l6 = new ListNode(9);
        ListNode l7 = new ListNode(9);
        ListNode l8 = new ListNode(9);
        ListNode l9 = new ListNode(9);
        ListNode l10 = new ListNode(9);
        
        l1.next = l2;
        l2.next = l3;
        l3.next = l4;
        l4.next = l5;
        l5.next = l6;
        l6.next = l7;
        l7.next = l8;
        l8.next = l9;
        l9.next = l10;
        
        System.out.println(addTwoNumbers(n1, l1).toString());
    }
    
    public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if (l1 == null || l2 == null) {
            return null;
        }
        
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;
        int carry = 0;
        
        while (l1 != null || l2 != null || carry == 1) {
            int sum = carry;
            if (l1 != null) {
                sum += l1.val;
                l1 = l1.next;
            }
            
            if (l2 != null) {
                sum += l2.val;
                l2 = l2.next;
            }
            
            carry = sum / 10;
            
            // create a new node and add it to the tail.
            ListNode cur = new ListNode(sum % 10);
            tail.next = cur;
            tail = tail.next;
        }
        
        return dummy.next;
    }
}

代码：

AddTwoNumbers