Leetcode: 2.Add Two Numbers

摘要： 這是崔斯特的第八十九篇原創文章 2. Add Two Numbers 難度: Medium 刷題內容 原題連線： https://leetcode.com/problems/add-two-numbers/description/> 內容描述 Y...

這是崔斯特的第八十九篇原創文章

2. Add Two Numbers

難度: Medium

刷題內容

原題連線： https://leetcode.com/problems/add-two-numbers/description/>

內容描述

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.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

解題方案

思路 1

• 時間複雜度: O(N)
• 空間複雜度: O(N)

將兩個連結串列轉為列表，反轉之後相加，再反轉，最後將結果轉為連結串列。

# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None

class Solution:
def addTwoNumbers(self, l1: ListNode, l2: ListNode)-> ListNode:
if not l1:
return l2
if not l2:
return l1

val1, val2 = [l1.val], [l2.val]
while l1.next:
val1.append(l1.next.val)
l1 = l1.next
while l2.next:
val2.append(l2.next.val)
l2 = l2.next

num1 = ''.join([str(i) for i in val1[::-1]])
num2 = ''.join([str(i) for i in val2[::-1]])

tmp = str(int(num1) + int(num2))[::-1]
res = ListNode(int(tmp[0]))
run_res = res
for i in range(1, len(tmp)):
run_res.next = ListNode(int(tmp[i]))
run_res = run_res.next
return res

result：

Runtime: 124 ms, faster than 33.23% of Python3 online submissions for Add Two Numbers.

Memory Usage: 13.4 MB, less than 5.21% of Python3 online submissions for Add Two Numbers.

這是最簡單的解法，但是面試官往往不會滿足，他肯定還會問，你還有簡單的解法嗎？空間複雜度可以再低一些嗎？

思路2

• 時間複雜度: O(N)
• 空間複雜度: O(1)

可以使用遞迴，每次算一位的相加

# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None

class Solution:
def addTwoNumbers(self, l1: ListNode, l2: ListNode)-> ListNode:
if not l1 and not l2:
return 
elif not (l1 and l2):
return l1 or l2
else:
if l1.val + l2.val < 10:
l3 = ListNode(l1.val+l2.val)
l3.next = self.addTwoNumbers(l1.next, l2.next)
else:
l3 = ListNode(l1.val+l2.val-10)
l3.next = self.addTwoNumbers(l1.next, self.addTwoNumbers(l2.next, ListNode(1)))
return l3

Result:

Runtime: 116 ms, faster than 42.88% of Python3 online submissions for Add Two Numbers.
Memory Usage: 13.3 MB, less than 5.21% of Python3 online submissions for Add Two Numbers.