Question

2. Add Two Numbers

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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

Solution 1: Simulation

We traverse two linked lists $l_1$ and $l_2$ at the same time, and use the variable $carry$ to indicate whether there is a carry.

Each time we traverse, we take out the current bit of the corresponding linked list, calculate the sum with the carry $carry$, and then update the value of the carry. Then we add the current bit to the answer linked list. If both linked lists are traversed, and the carry is $0$, the traversal ends.

Finally, we return the head node of the answer linked list.

The time complexity is $O(\max (m, n))$, where $m$ and $n$ are the lengths of the two linked lists. We need to traverse the entire position of the two linked lists, and each position only needs $O(1)$ time. Ignoring the space consumption of the answer, the space complexity is $O(1)$.

# Definition for singly-linked list.
# class ListNode:
#   def __init__(self, val=0, next=None):
#     self.val = val
#     self.next = next
class Solution:
  def addTwoNumbers(
    self, l1: Optional[ListNode], l2: Optional[ListNode]
  ) -> Optional[ListNode]:
    dummy = ListNode()
    carry, curr = 0, dummy
    while l1 or l2 or carry:
      s = (l1.val if l1 else 0) + (l2.val if l2 else 0) + carry
      carry, val = divmod(s, 10)
      curr.next = ListNode(val)
      curr = curr.next
      l1 = l1.next if l1 else None
      l2 = l2.next if l2 else None
    return dummy.next

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *   int val;
 *   ListNode next;
 *   ListNode() {}
 *   ListNode(int val) { this.val = val; }
 *   ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
  public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode dummy = new ListNode(0);
    int carry = 0;
    ListNode cur = dummy;
    while (l1 != null || l2 != null || carry != 0) {
      int s = (l1 == null ? 0 : l1.val) + (l2 == null ? 0 : l2.val) + carry;
      carry = s / 10;
      cur.next = new ListNode(s % 10);
      cur = cur.next;
      l1 = l1 == null ? null : l1.next;
      l2 = l2 == null ? null : l2.next;
    }
    return dummy.next;
  }
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *   int val;
 *   ListNode *next;
 *   ListNode() : val(0), next(nullptr) {}
 *   ListNode(int x) : val(x), next(nullptr) {}
 *   ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
  ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
    ListNode* dummy = new ListNode();
    int carry = 0;
    ListNode* cur = dummy;
    while (l1 || l2 || carry) {
      int s = (l1 ? l1->val : 0) + (l2 ? l2->val : 0) + carry;
      carry = s / 10;
      cur->next = new ListNode(s % 10);
      cur = cur->next;
      l1 = l1 ? l1->next : nullptr;
      l2 = l2 ? l2->next : nullptr;
    }
    return dummy->next;
  }
};

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *   Val int
 *   Next *ListNode
 * }
 */
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
  dummy := &ListNode{}
  carry := 0
  cur := dummy
  for l1 != nil || l2 != nil || carry != 0 {
    s := carry
    if l1 != nil {
      s += l1.Val
    }
    if l2 != nil {
      s += l2.Val
    }
    carry = s / 10
    cur.Next = &ListNode{s % 10, nil}
    cur = cur.Next
    if l1 != nil {
      l1 = l1.Next
    }
    if l2 != nil {
      l2 = l2.Next
    }
  }
  return dummy.Next
}

/**
 * Definition for singly-linked list.
 * class ListNode {
 *   val: number
 *   next: ListNode | null
 *   constructor(val?: number, next?: ListNode | null) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 *   }
 * }
 */

function addTwoNumbers(l1: ListNode | null, l2: ListNode | null): ListNode | null {
  const dummy = new ListNode();
  let cur = dummy;
  let sum = 0;
  while (l1 != null || l2 != null || sum !== 0) {
    if (l1 != null) {
      sum += l1.val;
      l1 = l1.next;
    }
    if (l2 != null) {
      sum += l2.val;
      l2 = l2.next;
    }
    cur.next = new ListNode(sum % 10);
    cur = cur.next;
    sum = Math.floor(sum / 10);
  }
  return dummy.next;
}

// Definition for singly-linked list.
// #[derive(PartialEq, Eq, Clone, Debug)]
// pub struct ListNode {
//   pub val: i32,
//   pub next: Option<Box<ListNode>>
// }
//
// impl ListNode {
//   #[inline]
//   fn new(val: i32) -> Self {
//   ListNode {
//     next: None,
//     val
//   }
//   }
// }
impl Solution {
  pub fn add_two_numbers(
    mut l1: Option<Box<ListNode>>,
    mut l2: Option<Box<ListNode>>
  ) -> Option<Box<ListNode>> {
    let mut dummy = Some(Box::new(ListNode::new(0)));
    let mut cur = &mut dummy;
    let mut sum = 0;
    while l1.is_some() || l2.is_some() || sum != 0 {
      if let Some(node) = l1 {
        sum += node.val;
        l1 = node.next;
      }
      if let Some(node) = l2 {
        sum += node.val;
        l2 = node.next;
      }
      cur.as_mut().unwrap().next = Some(Box::new(ListNode::new(sum % 10)));
      cur = &mut cur.as_mut().unwrap().next;
      sum /= 10;
    }
    dummy.unwrap().next.take()
  }
}

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *   this.val = (val===undefined ? 0 : val)
 *   this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function (l1, l2) {
  const dummy = new ListNode();
  let carry = 0;
  let cur = dummy;
  while (l1 || l2 || carry) {
    const s = (l1?.val || 0) + (l2?.val || 0) + carry;
    carry = Math.floor(s / 10);
    cur.next = new ListNode(s % 10);
    cur = cur.next;
    l1 = l1?.next;
    l2 = l2?.next;
  }
  return dummy.next;
};

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *   public int val;
 *   public ListNode next;
 *   public ListNode(int val=0, ListNode next=null) {
 *     this.val = val;
 *     this.next = next;
 *   }
 * }
 */
public class Solution {
  public ListNode AddTwoNumbers(ListNode l1, ListNode l2) {
    ListNode dummy = new ListNode();
    int carry = 0;
    ListNode cur = dummy;
    while (l1 != null || l2 != null || carry != 0) {
      int s = (l1 == null ? 0 : l1.val) + (l2 == null ? 0 : l2.val) + carry;
      carry = s / 10;
      cur.next = new ListNode(s % 10);
      cur = cur.next;
      l1 = l1 == null ? null : l1.next;
      l2 = l2 == null ? null : l2.next;
    }
    return dummy.next;
  }
}

/**
 * Definition for a singly-linked list.
 * class ListNode {
 *   public $val = 0;
 *   public $next = null;
 *   function __construct($val = 0, $next = null) {
 *     $this->val = $val;
 *     $this->next = $next;
 *   }
 * }
 */
class Solution {
  /**
   * @param ListNode $l1
   * @param ListNode $l2
   * @return ListNode
   */
  function addTwoNumbers($l1, $l2) {
    $dummy = new ListNode(0);
    $current = $dummy;
    $carry = 0;

    while ($l1 !== null || $l2 !== null) {
      $x = $l1 !== null ? $l1->val : 0;
      $y = $l2 !== null ? $l2->val : 0;

      $sum = $x + $y + $carry;
      $carry = (int) ($sum / 10);
      $current->next = new ListNode($sum % 10);
      $current = $current->next;

      if ($l1 !== null) {
        $l1 = $l1->next;
      }

      if ($l2 !== null) {
        $l2 = $l2->next;
      }
    }

    if ($carry > 0) {
      $current->next = new ListNode($carry);
    }

    return $dummy->next;
  }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *   public var val: Int
 *   public var next: ListNode?
 *   public init() { self.val = 0; self.next = nil; }
 *   public init(_ val: Int) { self.val = val; self.next = nil; }
 *   public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
 * }
 */
class Solution {
  func addTwoNumbers(_ l1: ListNode?, _ l2: ListNode?) -> ListNode? {
    var dummy = ListNode.init()
    var carry = 0
    var l1 = l1
    var l2 = l2
    var cur = dummy
    while l1 != nil || l2 != nil || carry != 0 {
      let s = (l1?.val ?? 0) + (l2?.val ?? 0) + carry
      carry = s / 10
      cur.next = ListNode.init(s % 10)
      cur = cur.next!
      l1 = l1?.next
      l2 = l2?.next
    }
    return dummy.next
  }
}

# Definition for singly-linked list.
# class ListNode
#   attr_accessor :val, :next
#   def initialize(val = 0, _next = nil)
#     @val = val
#     @next = _next
#   end
# end
# @param {ListNode} l1
# @param {ListNode} l2
# @return {ListNode}
def add_two_numbers(l1, l2)
  dummy = ListNode.new()
  carry = 0
  cur = dummy
  while !l1.nil? || !l2.nil? || carry > 0
    s = (l1.nil? ? 0 : l1.val) + (l2.nil? ? 0 : l2.val) + carry
    carry = s / 10
    cur.next = ListNode.new(s % 10)
    cur = cur.next
    l1 = l1.nil? ? l1 : l1.next
    l2 = l2.nil? ? l2 : l2.next
  end
  dummy.next
end

#[
  # Driver code in the solution file
  # Definition for singly-linked list.
  type
  Node[int] = ref object
    value: int
    next: Node[int]

  SinglyLinkedList[T] = object
    head, tail: Node[T]
]#

# More efficient code churning ...
proc addTwoNumbers(l1: var SinglyLinkedList, l2: var SinglyLinkedList): SinglyLinkedList[int] =
  var
  aggregate: SinglyLinkedList
  psum: seq[char]
  temp_la, temp_lb: seq[int]

  while not l1.head.isNil:
  temp_la.add(l1.head.value)
  l1.head = l1.head.next

  while not l2.head.isNil:
  temp_lb.add(l2.head.value)
  l2.head = l2.head.next

  psum = reversed($(reversed(temp_la).join("").parseInt() + reversed(temp_lb).join("").parseInt()))
  for i in psum: aggregate.append(($i).parseInt())

  result = aggregate