動機

要記得進位

Problem

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 contains a single digit. Add the two numbers and return the sum as a linked list.

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

 

Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4]Output: [7,0,8]Explanation: 342 + 465 = 807.

Example 2:

Input: l1 = [0], l2 = [0]Output: [0]

Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]Output: [8,9,9,9,0,0,0,1]

 

Constraints:

  • The number of nodes in each linked list is in the range [1, 100].
  • 0 <= Node.val <= 9
  • It is guaranteed that the list represents a number that does not have leading zeros.

Sol

主要就是進位，一直走下去 要注意是最後還有沒有進位，如果有要多一個node

def size(l):
    ret = 0
    while l:
        l = l.next
        ret += 1
    return ret

class Solution:
    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
        car = 0
        # premise: l1.len > l2.len
        if size(l2) > size(l1):
            l1, l2 = [l2, l1]
        ret = l1
        prev = l1
        while l1:
            tmp = l1.val + car + (l2.val if l2 else 0)
            l1.val = tmp % 10
            car = tmp // 10
            
            prev = l1
            l1 = l1.next if l1 else None
            l2 = l2.next if l2 else None
        if car != 0:
            prev.next = ListNode(car)
        return ret