Leetcode – Add Two Numbers (Java)

 

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

Java Solution

/*
2 -> 4 -> 3
5 -> 6 -> 4
7    0    8
*/
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode fake = new ListNode(0);
    ListNode p = fake;
 
    ListNode p1 = l1;
    ListNode p2 = l2;
 
    int carry = 0;
    while(p1!=null || p2!=null){
        int sum = carry;
        if(p1!=null){
            sum += p1.val;
            p1 = p1.next;
        }
 
        if(p2!=null){
            sum += p2.val;
            p2 = p2.next;
        }
 
        if(sum>9){
            carry=1;
            sum = sum-10;
        }else{
            carry = 0;
        }
 
        ListNode l = new ListNode(sum);
        p.next = l;
        p = p.next;
    }
 
    //don't forget check the carry value at the end
    if(carry > 0){
        ListNode l = new ListNode(carry);
        p.next = l;
    }
    return fake.next;
}

What if the digits are stored in regular order instead of reversed order?

Answer: We can simple reverse the list, calculate the result, and reverse the result.

Related posts:

  1. LeetCode – Reverse Nodes in k-Group (Java)
  2. LeetCode – Plus One (Java)
  3. LeetCode – Plus One Linked List (Java)
  4. LeetCode – Merge Two Sorted Lists (Java)
Category >> Algorithms  
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  • Bonsai

    Right. The first node can be detached, by fake.next = null. But it does not affect the correctness of the code.


    ListNode result = fake.next;
    fake.next = null;

  • Chaitanya GOPIREDDY

    First node in the solution will always be 0.

  • Marina


    package structures;

    public class AddTwoNumbers {

    public LinkedList calculate(LinkedList first, LinkedList second) {
    int carryOver = 0;
    LinkedList result = new LinkedList();
    Node firstCurrent = first.getHead();
    Node secondCurrent = second.getHead();

    while (firstCurrent != null && secondCurrent != null) {
    int res = firstCurrent.getVal() + secondCurrent.getVal() + carryOver;

    firstCurrent = firstCurrent.getNext();
    secondCurrent = secondCurrent.getNext();
    result.add(res % 10);
    carryOver = carry(res);
    }

    while (firstCurrent != null) {
    int res = carryOver + firstCurrent.getVal();
    result.add(res% 10);
    carryOver = carry(res);
    }

    while (secondCurrent != null) {
    int res = carryOver + secondCurrent.getVal();
    result.add(res% 10);
    carryOver = carry(res);
    }

    return result;
    }

    private int carry(int res) {
    int carryOver;
    if (res > 9) {
    carryOver = 1;
    } else {
    carryOver = 0;
    }
    return carryOver;
    }
    }

  • Работа в интернете

  • Kanak Lata

    public static LinkedList addTwo(LinkedList l1,LinkedList l2){
    LinkedList res= new LinkedList();
    int carry=0;
    while(l1.size()!=0&&l2.size()!=0){
    int x=(int)l1.removeFirst()+(int)l2.removeFirst()+carry;
    int r=(x)%10;
    carry=(x)/10;
    res.add(r);}
    while(l1.size()!=0){
    int x=(int)l1.removeFirst()+carry;
    int r=(x)%10;
    carry=(x)/10;
    res.add(r);}
    while(l2.size()!=0){
    int x=(int)l2.removeFirst()+carry;
    int r=(x)%10;
    carry=(x)/10;
    res.add(r);}
    if(carry==1)
    res.add(carry);
    return res;}

  • Peter


    public static LinkedList addNumbers(LinkedList l1, LinkedList l2){
    LinkedList result = new LinkedList();
    int value=0;
    int carry=0;
    Node iterator1 = l1.getHead();
    Node iterator2 = l2.getHead();

    while(iterator1!=null && iterator2 !=null){
    value=iterator1.getValue()+iterator2.getValue();
    //checking if it is the last node of the list
    // and if the sum needs another node.
    if(iterator1.getPointer()==null && value+carry>9){
    result.insertLast((value+carry)%10);
    result.insertLast(1);
    }
    else{
    result.insertLast((value+carry)%10);
    if(value+carry>9)
    carry=1;
    else
    carry=0;
    }
    iterator1=iterator1.getPointer();
    iterator2=iterator2.getPointer();
    }
    return result;
    }

  • Peter

    public static LinkedList addNumbers(LinkedList l1, LinkedList l2){
    LinkedList result = new LinkedList();
    int value=0;
    int carry=0;
    Node iterator1 = l1.getHead();
    Node iterator2 = l2.getHead();

    while(iterator1!=null && iterator2 !=null){
    value=iterator1.getValue()+iterator2.getValue();
    //checking if it is the last node of the list
    // and if the sum needs another node.
    if(iterator1.getPointer()==null && value+carry>9){
    result.insertLast((value+carry)%10);
    result.insertLast(1);
    }
    else{
    result.insertLast((value+carry)%10);
    if(value+carry>9)
    carry=1;
    else
    carry=0;
    }
    iterator1=iterator1.getPointer();
    iterator2=iterator2.getPointer();
    }
    return result;
    }

  • Arrow

    This might not work in the case when you add 1 and 0. Probably do if (p1.val+p2.val )>9.

  • Peeyush Chandel


    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    int carry = 0;
    int num = 0;
    ListNode r = null;
    ListNode head = null;
    while (l1 != null || l2 != null) {
    if (l1 != null && l2 != null) {
    num = l1.val + l2.val;
    l1 = l1.next;
    l2 = l2.next;
    } else if (l1 != null) {
    num = l1.val;
    l1 = l1.next;
    } else if (l2 != null) {
    num = l2.val;
    l2 = l2.next;
    }
    if ((num + carry) >= 10) {
    num = (num + carry) - 10;
    carry = 1;
    } else {
    num = num + carry;
    carry = 0;
    }
    if (r == null) {
    r = new ListNode(num);
    head = r;
    } else {
    ListNode l = new ListNode(num);
    r.next = l;
    r = r.next;
    }

    }
    if (carry != 0) {
    ListNode l = new ListNode(carry);
    r.next = l;
    }
    return head;
    }

  • Udaydeep Thota

    You could do it, but its not a good practice to navigate the list with the head pointers.

  • Udaydeep Thota

    Works for all the cases, you are just changing the order of numbers and adding it in the same way. Eg: 490 +829 = 1319. Using this method gives 094+928 = 9131 which is correct (Reverse of 9131 is 1319)

  • Milan

    A slightly simpler version,

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    int reminder = 0, sum = 0, nextDigit = 0;
    ListNode dummyNode = new ListNode(0);
    ListNode T = dummyNode;
    while (l1 !=null && l2 != null){
    sum = l1.val+l2.val+reminder;
    nextDigit = sum > 9 ? sum-10 : sum;
    reminder = sum > 9 ? 1 : 0;
    T.next = new ListNode(nextDigit);
    l1=l1.next;
    l2=l2.next;
    T=T.next;
    }
    while (l1 !=null){
    sum = l1.val+reminder;
    nextDigit = sum > 9 ? sum-10 : sum;
    reminder = sum > 9 ? 1 : 0;
    l1=l1.next;
    T.next = new ListNode(nextDigit);
    T=T.next;
    }
    while (l2 !=null){
    sum = l2.val+reminder;
    nextDigit = sum > 9 ? sum-10 : sum;
    reminder = sum > 9 ? 1 : 0;
    l2=l2.next;
    T.next = new ListNode(nextDigit);
    T=T.next;
    }
    if (reminder != 0 )
    T.next = new ListNode(reminder);
    return dummyNode.next;
    }

  • Rahul

    Thanks for the code.. Logically its correct. The only concern is about adding a 4 digit number to a 3 digit number in regular order

  • Jayce Kim

    I am also curious about that. Did you figure it out?

  • random

    Bunch of looser have you guys tested it once ?
    it wasted my 50 min of time.

  • Zach

    Doesn’t work if first two numbers added together are >= 10 does it?

  • Wonderer

    I wonder, why do you need a new name for l1 and l2 and don’t just use l1 and l2?

  • In this case an additional ListNode is added at the end of resulting sum. The following excerpt solves the case:

    if(carry==1)
    p3.next=new ListNode(1);

  • Mohit

    What if you get 4 digits after adding two 3-digit numbers?