2. Add Two Numbers LeetCode

Time complexity: O(N)

/**
	* 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* root = new ListNode(0);
	ListNode* ret = root;
	int carry = 0;
	while(l1 or l2){
	int sum = carry;
	if(l1){
	sum+= l1->val;
	l1=l1->next;
	}
	if(l2){
	sum+= l2->val;
	l2=l2->next;
	}
	carry = sum / 10;
	sum = sum % 10;
	ret->next = new ListNode(sum);
	ret = ret->next;
	}
	while(l1){
	int sum = carry + l1->val;
	l1=l1->next;
	carry = sum / 10;
	sum = sum % 10;
	ret->next = new ListNode(sum);
	ret = ret->next;
	}
	while(l2){
	int sum = carry + l2->val;
	l2=l2->next;
	carry = sum / 10;
	sum = sum % 10;
	ret->next = new ListNode(sum);
	ret = ret->next;
	}
	if(carry){
	ret->next = new ListNode(carry);
	ret = ret->next;
	}
	return root->next;
	}
	};