2. Add Two Numbers

Description

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.

Solutions

I Iterative

Traverse two linked lists at the same time iteratively, and add the corresponding digits and carry.

• Time complexity: \(O(max(m, n))\), where \(m\) and \(n\) are the lengths of the two linked lists.
• Space complexity: \(O(1)\).

type ListNode struct {
    Val  int
    Next *ListNode
}

func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
    dummy, isCarry := ListNode{}, false
    p := &dummy

    for l1 != nil || l2 != nil || isCarry {
        val := 0
        if isCarry {
            val += 1
        }
        if l1 != nil {
            val += l1.Val
            l1 = l1.Next
        }
        if l2 != nil {
            val += l2.Val
            l2 = l2.Next
        }

        isCarry = val > 9
        p.Next = &ListNode{Val: val % 10}
        p = p.Next
    }

    return dummy.Next
}

II Recursive

Traverse two linked lists at the same time recursively, and add the corresponding digits and carry.

• Time complexity: \(O(max(m, n))\), where \(m\) and \(n\) are the lengths of the two linked lists.
• Space complexity: \(O(max(m, n))\), where \(m\) and \(n\) are the lengths of the two linked lists.

type ListNode struct {
    Val  int
    Next *ListNode
}

func addTwoNumbersRecursion(l1 *ListNode, l2 *ListNode, isCarry bool) *ListNode {
    if l1 == nil && l2 == nil && !isCarry {
        return nil
    }
    node, val := ListNode{}, 0
    if isCarry {
        val += 1
    }
    if l1 != nil {
        val += l1.Val
        l1 = l1.Next
    }
    if l2 != nil {
        val += l2.Val
        l2 = l2.Next
    }

    isCarry = val > 9
    node.Val = val % 10
    node.Next = addTwoNumbersRecursion(l1, l2, isCarry)
    return &node
}

func addTwoNumbersII(l1 *ListNode, l2 *ListNode) *ListNode {
    return addTwoNumbersRecursion(l1, l2, false)
}