LeetCode 2. 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

 

---

【题目分析】

用两个链表来表示两个非负数的逆序拍列，每个节点包含一个数字。例如：2 -> 4 -> 3代表342，5 -> 6 -> 4代表465,342+465 = 807。求这两个数的和，并用同样的方式返回。

---

【思路】

两数按位相加，只要处理好向后进位即可。当前相加结果 = 链表1数字+链表2数字+前一个进位。

---

【java代码】

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1==null) return l2;
        if(l2==null) return l1;
        
        ListNode l3 = new ListNode(0);
        ListNode tail = l3;
        
        int flag = 0;
        while(l1!=null || l2!=null){
            int num1 = 0, num2 = 0;
            if(l1 != null){
                num1 = l1.val;
                l1 = l1.next;
            }
            if(l2 != null){
                num2 = l2.val;
                l2 = l2.next;
            }
            int num = num1 + num2 + flag;
            flag = num / 10;
            num = num % 10;
            ListNode node = new ListNode(num);
            tail.next = node;
            tail = node;
        }
        if(flag == 1){
            ListNode node = new ListNode(1);
            tail.next = node;
            tail = node;
        }
        tail.next = null;
        return l3.next;
    }
}