Notebook

2. Add Two Numbers

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.

Source

In [1]:

# Definition for singly-linked list.
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

Code

In [2]:

def add_two_numbers(l1, l2):
    """iterative approach."""
    carry = 0
    answer = curr_sol = ListNode(carry)
    while l1 or l2 or carry != 0:
        tmp = carry
        if l1:
            tmp += l1.val
            l1 = l1.next
        if l2:
            tmp += l2.val
            l2 = l2.next
        val = tmp % 10
        carry = tmp // 10
        curr_sol.next = ListNode(val)
        curr_sol = curr_sol.next
    return answer.next

Follow up:

Solve it both recursively and iteratively.

Code

In [ ]:

def add_two_numbers(l1, l2):
    """recursive approach."""
    def helper(l1,l2,carry):
        if l1 or l2 or carry != 0:
            tmp = carry
            if l1:
                tmp += l1.val
                l1 = l1.next
            if l2:
                tmp += l2.val
                l2 = l2.next
            val = tmp % 10
            carry = tmp // 10
            answer = ListNode(val) 
            answer.next = helper(l1, l2, carry)    
            return answer
        return None

    return helper(l1,l2,0)