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 contain a single digit. Add the two numbers and return it as a linked list.

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

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

My code :

java:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1==null) return l2;
        if(l2==null) return l1;

        ListNode head = new ListNode(0);
        ListNode p = head;

        int carry = 0;

        while(l1!=null || l2!=null || carry!=0){
            if(l1!=null){
                carry += l1.val;
                l1 = l1.next;
            }
            if(l2!=null){
                carry += l2.val;
                l2 = l2.next;
            }
            p.next = new ListNode(carry%10);
            p = p.next;
            carry = carry/10; 
        }

        return head.next;
    }
}

Runtime: 1 ms

Optimized code

java:

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

Runtime: 0 ms

Better code

java:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummyHead = new ListNode(0);
        ListNode p = l1, q = l2, curr = dummyHead;
        int carry = 0;
        while (p != null || q != null) {
            int x = (p != null) ? p.val : 0;
            int y = (q != null) ? q.val : 0;
            int sum = carry + x + y;
            carry = sum / 10;
            curr.next = new ListNode(sum % 10);
            curr = curr.next;
            if (p != null) p = p.next;
            if (q != null) q = q.next;
        }
        if (carry > 0) {
            curr.next = new ListNode(carry);
        }
        return dummyHead.next;
    }
}

Runtime: 0 ms

• Time complexity : O(max(m, n)). Assume that m and n represents the length of l1 and l2 respectively, the algorithm above iterates at most max(m, n) times.

• Space complexity : O(max(m, n)). The length of the new list is at most max(m, n) + 1.

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