Add Two Numbers

1. Problem Description

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each node stores a single digit.

Add the two numbers and return the sum as a linked list in the same format.

You may assume the two numbers do not contain leading zeros, except for the number 0 itself.

2. Approach

1. Approach 1
   • Ignore integer overflow
   • Convert the two linked lists to integers, add them, then convert the result back to a linked list
   • This implementation reads the linked lists with the most significant digit first, which is inconsistent with the reverse-order representation defined by this LeetCode problem; it is retained only as a historical implementation
2. Approach 2
   • The digits are stored in reverse order, so reverse the corresponding linked-list representation
   • Add from right to left to obtain the final result
   • Reverse the result
3. Approach 3
   1. Same as Add Strings

3. Implementation

3.1. Ignore Integer Overflow

// Time: O(N)
// Space: O(1)
func addInList(head1 *ListNode, head2 *ListNode) *ListNode {
	if head1 == nil {
		return head2
	}
	if head2 == nil {
		return head1
	}

	val1 := 0
	for head1 != nil {
		val1 = val1*10 + head1.Val
		head1 = head1.Next
	}

	val2 := 0
	for head2 != nil {
		val2 = val2*10 + head2.Val
		head2 = head2.Next
	}

	sum := val1 + val2
	if sum == 0 {
		return &ListNode{Val: 0}
	}
	var head *ListNode
	for sum != 0 {
		current := sum % 10
		head = &ListNode{
			Val:  current,
			Next: head,
		}
		sum /= 10
	}

	return head
}

3.2. Stack

package main

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
	lst1 := listToSlice(l1)
	lst2 := listToSlice(l2)
	reverse(lst1)
	reverse(lst2)
	dummyHead := &ListNode{
	}
	i := len(lst1) - 1
	j := len(lst2) - 1
	jinwei := 0
	for i >= 0 || j >= 0 || jinwei > 0 {
		a := 0
		b := 0
		if i >= 0 {
			a = lst1[i]
			i--
		}
		if j >= 0 {
			b = lst2[j]
			j--
		}

		val := (a + b + jinwei) % 10
		dummyHead.Next = &ListNode{
			Val:  val,
			Next: dummyHead.Next,
		}

		jinwei = (a + b + jinwei) / 10
	}

	head := dummyHead.Next
	dummyHead.Next = nil
	return reverseList(head)
}

func reverseList(head *ListNode) *ListNode {
	if head == nil {
		return nil
	}

	var prev *ListNode
	current := head
	next := head.Next

	for current != nil {
		current.Next = prev
		prev = current
		current = next
		if next != nil {
			next = next.Next
		}
	}
	return prev
}

func reverse(lst []int) {
	left := 0
	right := len(lst) - 1
	for left < right {
		lst[left], lst[right] = lst[right], lst[left]
		left++
		right--
	}
}

func listToSlice(head *ListNode) []int {
	lst := make([]int, 0)
	current := head
	for current != nil {
		lst = append(lst, current.Val)
		current = current.Next
	}
	return lst
}

3.3. Traverse the Linked Lists

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
    if l1 == nil {
        return l2
    }
    if l2 == nil {
        return l1
    }

    h1 := l1
    h2 := l2
    dummyHead := &ListNode{Val:-1}
    d := dummyHead
    jinwei := 0
    for h1 != nil || h2 != nil || jinwei > 0 {
        sum := 0
        if h1 != nil {
            sum += h1.Val
            h1 = h1.Next
        }
        if h2 != nil {
            sum += h2.Val
            h2 = h2.Next
        }
        sum += jinwei
        d.Next = &ListNode{Val:sum%10}
        d = d.Next
        jinwei = sum / 10

    }

    return dummyHead.Next
}

4. References

• 2. Add Two Numbers - LeetCode
• Add Two Numbers Represented by Linked Lists - NowCoder