445. Add Two Numbers II

1. Question

445. Add Two Numbers II

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

2. Solution

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

class Solution:
    def reverseList(self, root):
        if root is None:
            return root
        head = root
        root = root.next
        head.next = None
        while root:
            tmp = root.next
            root.next = head
            head = root
            root = tmp
        return head

    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        l1 = self.reverseList(l1)
        l2 = self.reverseList(l2)
        ins, head_value = divmod(l1.val + l2.val, 10)
        head = ListNode(head_value)

        l1 = l1.next
        l2 = l2.next
        while l1 and l2:
            ins, node_value = divmod(l1.val + l2.val + ins, 10)
            node = ListNode(node_value)
            node.next = head
            head = node
            l1 = l1.next
            l2 = l2.next

        while l1:
            ins, node_value = divmod(l1.val + ins, 10)
            node = ListNode(node_value)
            node.next = head
            head = node
            l1 = l1.next

        while l2:
            ins, node_value = divmod(l2.val + ins, 10)
            node = ListNode(node_value)
            node.next = head
            head = node
            l2 = l2.next
        
        if ins:
            node = ListNode(1)
            node.next = head
            head = node
        return head

3. Complexity Analysis

Time Complexity : O(N)

Space Complexity: O(1)