leetcode练习2：两数相加

题目二

：

 

解答：

1、判断链表的长度

2、如何增加节点（调用类）

3、进位问题，如果进位的下一个节点为空，则必须新添加一个节点

# Definition for singly-linked list.

# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if self.getLength(l1) < self.getLength(l2): #保证l1永远比l2更长
            l1, l2 = l2, l1 
        #将未处理的链表的头节点赋值给head1和head2
        head1 = l1
        head2 = l2           
        while(head2):#执行加法
            head1.val += head2.val
            head1 = head1.next
            head2 = head2.next
        
        head1 = l1 #将已经相加的链表的头节点赋值给head1
        while(head1):
            if head1.val > 9:
                if head1.next:
                    head1.next.val += 1
                    head1.val -= 10
                else:
                    head1.next = ListNode(1)
                    head1.val -=  10
            head1 = head1.next
        return l1    
       
    def getLength(self, l):#计算链表长度
        length = 0
        while(l):
            length += 1
            l = l.next
        return length