LeetCode-2. Add Two Numbers

[ninehills]

20170714

题目

https://leetcode.com/problems/add-two-numbers/

You are given two non-empty linked lists representing two non-negative integers. 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.

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

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

注意点：列表给的就是反序的，是342+465 = 807

思路

• 链表遍历，另外进位只可能进1，不可能进更多的数
• 注意增加了一个RootNode，用它的next放第一个节点
• 需要添加辅助的函数和类定义用来测试和debug

解答

class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None
        
    def __str__(self):
        return "Node({0}->[{1}])".format(self.val, self.next)

           
def init_list(l):
    before = None
    for i in l[::-1]:
        n = ListNode(i)
        n.next = before
        before = n
    return n

class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        carry = 0
        root = before = ListNode(None)
        while l1 or l2 or carry:
            v1 = v2 = 0
            if l1:
                v1 = l1.val
                l1 = l1.next
            if l2:
                v2 = l2.val
                l2 = l2.next
            z = v1 + v2 + carry
            if z >= 10:
                x = z - 10
                carry = 1
            else:
                x = z
                carry = 0
            node = ListNode(x)
            before.next = node
            before = node
        return root.next

# [7,0,8]
print Solution().addTwoNumbers(init_list([2,4,3]), init_list([5,6,4]))