2. Add Two Numbers

Problem Statement

Pattern:

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Solution 1

Extra Linked List solution

public ListNode addTwoNumbers (ListNode l1, ListNode l2) {
	ListNode ans = new ListNode(-1), ansHead = ans;
	int carry = 0;
	while (l1 != null || l2 != null || carry !=0) {
		
		int first  =  (l1 != null) ? l1.val : 0;
		int second = (l2 != null) ? l2.val : 0;
		
		carry = carry + first + second;
		ans.next = new ListNode(carry%10);
		carry/=10;
		
		// increment LLs
		ans = ans.next;
		l1 = (l1!=null) ? l1.next : null;
		l2 = (l2!=null) ? l2.next : null;
	
	}
	return ansHead.next;
}

👆 this ones far better honestly. i spend so much time on 👇that, that i didnt have the heart to not note it down

Solution 2

Add sum into the second linked

public ListNode addTwoNumbers (ListNode l1, ListNode l2){
	// p1 and p2 are dummy head l1 and l2s
	ListNode p1 = new ListNode(-1, l1), p2 = new ListNode(-1, l2);
	int carry = 0;
	
	// while p1 and l2 left
	while(p1.next != null && p2.next != null) {
		carry = carry + p2.next.val + p1.next.val;
		p2.next.val = carry % 10;
		carry /=10;
		
		p1 = p1.next;
		p2 = p2.next;
	}
	
	// while p1 left
	while(p1.next != null) {
		carry = carry + p1.next.val;
		p2.next = new ListNode(carry%10);
		carry /=10;
		
		p1 = p1.next;
		p2 = p2.next;
	}
	
	// while l2 left
	while (p2.next != null) {
		carry = carry + p2.next.val;
		p2.next.val = carry % 10;
		carry /=10;
		p2 = p2.next;
	}
	
	// while carry left
	while(carry != 0) {
		p2.next = new ListNode(carry%10);
		carry /= 10;
		p2 = p2.next;
	}
	
	return l2;
}