力扣 | 2.两数相加

题目截图

 官方方法

总共有三个链表，l1,l2和最后的结果链表。l1和l2各自滑动对应链表各个位数支出相应的值，最后相加，将结果添加到新链表上。

唯一需要考虑的是数值相加后的进位。

先设置进位carry为0

结果相加的时候利用取模和取整来进位。

比如，8+7=15，各位留5，进位为1。

创建一个头结点head和一个移动结点p

当l1或l2存在时，新建一个结点new_point并赋值为0。然后进入判断：

若l1不存在，则只用将l2的值付给新链表。反之，亦然。若两个都存在，则当前位的值为（l1的值+l2的值）取模，进位为两者之和除10取整。，然后指针往后滑动，知道l1和l2都遍历完成

还需要注意全部遍历完成后可能还有进位，再添加一个结点，并赋值为1。

最后返回头结点。

代码如下

 def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        head = point =  ListNode(0)
        carry = 0

        while l1 or l2:
            new_point = ListNode(0)

            if not l1:
                sum_=l2.val+carry
                new_point.val = sum_ % 10
                carry = sum_ // 10
                l2 = l2.next
            elif not l2:
                sum_ = l1.val +carry
                new_point.val = sum_ % 10
                carry = sum_ // 10
                l1 = l1.next
            else:
                sum_ = l1.val + l2.val + carry
                new_point.val = sum_ % 10
                carry = sum_ // 10
                l1 = l1.next
                l2 = l2.next
            point.next = new_point
            point = point.next

        if carry:
            new_point = ListNode(1)
            point.next = new_point
        return head.next

完整测试代码

# Definition for singly-linked list.
from typing import Optional

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

class Solution:
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        head = point =  ListNode(0)
        carry = 0

        while l1 or l2:
            new_point = ListNode(0)

            if not l1:
                sum_=l2.val+carry
                new_point.val = sum_ % 10
                carry = sum_ // 10
                l2 = l2.next
            elif not l2:
                sum_ = l1.val +carry
                new_point.val = sum_ % 10
                carry = sum_ // 10
                l1 = l1.next
            else:
                sum_ = l1.val + l2.val + carry
                new_point.val = sum_ % 10
                carry = sum_ // 10
                l1 = l1.next
                l2 = l2.next
            point.next = new_point
            point = point.next

        if carry:
            new_point = ListNode(1)
            point.next = new_point
        return head.next

class Linked_List:
    def create_linked_list(self, nums):
        if len(nums) == 0:
            return None
        head = ListNode(nums[0])
        cur = head
        for i in range(1, len(nums)):
            cur.next = ListNode(nums[i])
            cur = cur.next
        return head

def print_linked_list(list_node):
    if list_node is None:
        return

    cur = list_node
    while cur:
        print(cur.val, '->', end=' ')
        cur = cur.next
    print('null')


class main:
    list_1 = [2,4,3]
    list_2 = [5,6,4]
    solution = Solution()
    link_list = Linked_List()
    head_1 = link_list.create_linked_list(list_1)
    head_2 = link_list.create_linked_list(list_2)
    answer = solution.addTwoNumbers(head_1, head_2)
    print_linked_list(answer)

if __name__ == '__main__':
    main()

pycharm上测试结果

 

简化代码版

简化代码版的思路和官方方法相差不大。

但是直接将l1和l2以及进位的问题合并考虑，并运用三元表达式简化代码量。

代码如下

class Solution:
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        head = p = ListNode(None)
        carry = 0
        while l1 or l2 or carry:
            carry += (l1.val if l1 else 0) + (l2.val if l2 else 0)
            p.next = ListNode(carry % 10)
            p = p.next
            carry = carry // 10
            l1 = l1.next if l1 else None
            l2 = l2.next if l2 else None
        return head.next