2. 两数相加

/**

• Definition for singly-linked list.
• public class ListNode {
• int val;
• ListNode next;
• ListNode(int x) { val = x; }
• }
  */

class Solution {

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode head = new ListNode();
    ListNode result = head;
    boolean equalTen = false;
    //需要考虑l1比l2长，或者l2比l1长
    while(l1 != null || l2 != null){
        result.next = new ListNode();
        result = result.next;
        if(l1 != null && l2 == null){
            result.val = l1.val;
        }else if(l1 == null && l2 != null){
            result.val = l2.val;
        }
        else{
            result.val = l1.val + l2.val;
        }
        if(equalTen){
            result.val += 1;
        }
        if(result.val >= 10){
            equalTen = true;
            result.val -= 10;
        }else{
            equalTen = false;
        }
        if(l1!=null){
            l1=l1.next;
        }
        if(l2!=null){
            l2=l2.next;
        }
    }
   //需要考虑最后一位相加大于9的情况
    if(equalTen){
        result.next = new ListNode();
        result = result.next;
        result.val = 1;
    }
    return head.next;
}

}