2. Add Two Numbers

Posted 2020-08-09 Machelsky

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

思路：从尾巴开始加数，和大于10就变carry和值。需要注意当一个list读完后，那个list的值就是0，只需要加另一个list。最后判断carry的值，如果是0就直接返回，是1的话还要补上一个node。

/**
 * 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) {
        ListNode dummy=new ListNode(-1);
        ListNode copy=dummy;
        int carry=0;
            while(l1!=null||l2!=null)
            {
                int va1=0;
                if(l1!=null)
                {
                    va1=l1.val;
                    l1=l1.next;
                }
                int va2=0;
                if(l2!=null)
                {
                    va2=l2.val;
                    l2=l2.next;
                }
                if((va1+va2+carry)>=10)
                {
                copy.next=new ListNode(va1+va2+carry-10);
                carry=1;
                }
                else
                {
                    copy.next=new ListNode(va1+va2+carry);
                    carry=0;
                }
                copy=copy.next;
            }
            if(carry==0)
            {
                copy.next=null;
            }
            else
            {
                copy.next=new ListNode(1);
            }
            return dummy.next;
    }
}