Daily LeetCode 445. Add Two Numbers II

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

Medium

问题描述

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:

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

题目分析

这一题与昨天的题目有一些不同，昨天的题目链表是从低位到高位排列的，而Add two numbers II是从高位向低位排列的，因此我们需要先将两个链表反转，再按昨天的解法处理，处理完之后，再将得到的链表反转输出。

如果不能对链表进行反转操作，我们可以通过栈来解决这个问题，分别遍历两个链表，将元素入栈，再对栈操作，采用与Add two numbers相同的解法，不同的是，构建链表时，我们需要采用头插法。

代码：

反转链表：

# 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:
        def reverse_node(l):
            if not l or not l.next:
                return l
            pre = None
            while l:
                tmp = l.next
                l.next = pre
                pre = l
                l = tmp
            return pre
        res = point = ListNode(0)
        extra = 0
        l1 = reverse_node(l1)
        l2 = reverse_node(l2)

        while l1 != None or l2 != None:
            l1_ele = l1.val if l1 != None else 0
            l2_ele = l2.val if l2 != None else 0
            sum = l1_ele + l2_ele + extra
            extra = 1 if sum >= 10 else 0
            point.next = ListNode(int(sum%10))
            point = point.next
            if l1 != None: l1 = l1.next
            if l2 != None: l2 = l2.next

        if extra:
            point.next = ListNode(1)

        return reverse_node(res.next) 

栈：

# 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
        """
        stack1 = []
        stack2 = []
        while l1:
            stack1.append(l1.val)
            l1 = l1.next
        while l2:
            stack2.append(l2.val)
            l2 = l2.next
				res = ListNode()
        extra = 0
        
        while stack1 and stack2:
            sum = stack1.pop() + stack2.pop() + extra
            extra = 1 if sum >= 10 else 0
            tmp = res
						res = ListNode(int(sum % 10))
            res.next = tmp
            
        l = stack1 if stack1 else stack2
        
        while l:
            sum = l.pop() + extra
            extra = 1 if sum >= 10 else 0
            tmp = res
            res = ListNode(int(sum % 10))
            res.next = tmp
            
        if extra:
            tmp = res
            res = ListNode(1)
            res.next = tmp    
        return res.next