You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example: 

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

思路

这道题目可以采用简单的链表操作来搞定。题目说要求不能反转链表，这样无形中就会增加我们算法的复杂度。

我们采用竖式加法运算操作，

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  7 2 4 3
+   5 6 4
= 7 8 0 7

我们首先可以遍历一遍得到链表的长度。再进行O(n^2)的操作，多次遍历单链表，从个位开始，从后往前进行加法运算。注意进位即可。

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# 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
        """
        length1 = 0
        length2 = 0
        l1cur = l1
        l2cur = l2
        while(l1cur.next):
            length1 += 1
            l1cur = l1cur.next
        while(l2cur.next):
            length2 += 1
            l2cur = l2cur.next
        
        res = []
        carry = 0
        l1val = l1cur.val
        l2val = l2cur.val
        while(length1 >= 0 or length2 >= 0):
            tmpsum = l1val + l2val + carry
            if tmpsum >= 10:
                tmpsum = tmpsum - 10
                carry = 1
            else:
                carry = 0
            cur = ListNode(tmpsum)
            res.append(cur)
            
            l1cur = l1
            l2cur = l2
            
            if length1 > 0:
                for i in range(length1-1):
                    l1cur = l1cur.next
                l1val = l1cur.val
            else:
                l1val = 0
            
            if length2 > 0:
                for i in range(length2-1):
                    l2cur = l2cur.next
                l2val = l2cur.val
            else:
                l2val = 0
                
            length1 -= 1
            length2 -= 1
        
        if (carry):
            cur = ListNode(carry)
            res.append(cur)
        
        length = len(res)
        if (length == 0):
            return None
        elif (length == 1):
            return res[0]
        else:
            head = res[-1]
            for i in range(length-1, 0, -1):
                res[i].next = res[i-1]
            return head