[LeetCode] Add Two Numbers II

 

 

Problem

 

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Solution

 

We can easily see that this problem is just elementary school math, except that the digits are divided into nodes in the linked lists.

Turn each list into an integer, then add the two numbers up. 

Finally, turn the sum into a linked list by iterating through the digits.

 

Time Complexity

O(N1+N2), where N1 and N2 are the numbers of elements in each input list.

 

 

Space Complexity

O(1)

 

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if not l1 and not l2:
        	return None
        
        l1_num = l2_num = 0
        while l1:
        	l1_num = l1_num * 10 + l1.val
            l1 = l1.next
        
        while l2:
        	l2_num = l2_num * 10 + l2.val
            l2 = l2.next
        
       	root = curr = ListNode(0)
        for n in str(l1_num + l2_num):
        	curr.next = ListNode(int(n))
            curr = curr.next
        
        return root.next