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Problem

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.

Examples

Example 1:

(2 -> 4 -> 3) + (5 -> 6 -> 4) => 7 -> 0 -> 8

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]

Explanation:  342 + 465 = 807

Solution

This problem is straight forward, but consider the cases:

We should not forget the carry, especially when we have one more digit just for the carry. (e.g., 5 + 7 = 12, the ’1′ in 12 is due to the carry from last digit)
Two numbers might not have same number of digits. (e.g., 1 + 123 are one digit and three digit numbers)
As the link list are reversed that of number, we have taken care of adding it from right to left i.e. least significant digit to most significant digit.

add-two-nums-as-linked-list-reversed-expl1.excalidraw

Method 1 - OR in While Loop

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {  
    int carry = 0;  
  
    ListNode newHead = new ListNode(0);  
    ListNode l3 = newHead;  
  
    while (l1 != null || l2 != null) {  
        if (l1 != null) {  
            carry += l1.val;  
            l1 = l1.next;  
        }  
  
        if (l2 != null) {  
            carry += l2.val;  
            l2 = l2.next;  
        }  
  
        l3.next = new ListNode(carry % 10);  
        l3 = l3.next;  
        carry /= 10;  
    }  
	// important edge case
    if (carry > 0) {  
        l3.next = new ListNode(carry);  
    }  
  
    return newHead.next;  
}

Note that we are calling - if (p1!=null) and if(p2!=null) - we can optimize a little, but code is not that clean.

Method 2 - And in While Loop

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
	ListNode dummy = new ListNode(-1);
	ListNode dummyHead = dummy;
	int extra = 0;
	while (l1 != null && l2 != null) {
		dummy.next = new ListNode((l1.val + l2.val + extra) % 10);
		extra = l1.val + l2.val + extra > 9 ? 1 : 0;
		dummy = dummy.next;
		l1 = l1.next;
		l2 = l2.next;
	}
	ListNode n = l1 != null ? l1 : l2;
	while (n != null) {
		dummy.next = new ListNode((n.val + extra) % 10);
		extra = n.val + extra > 9 ? 1 : 0;
		n = n.next;
		dummy = dummy.next;
	}
	if (extra == 1) {
		dummy.next = new ListNode(1);
	}
	return dummyHead.next;
}

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