[LeetCode] 2. Add Two Numbers - 문제풀이

Description

주어진 두 링크드 리스트의 역순으로 읽은 숫자의 합을 역순으로 반환하는 문제입니다.

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.

Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

Example 2:

Input: l1 = [0], l2 = [0]
Output: [0]

Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
Output: [8,9,9,9,0,0,0,1]

Constraints:

• The number of nodes in each linked list is in the range [1, 100].
• 0 <= Node.val <= 9
• It is guaranteed that the list represents a number that does not have leading zeros.

Solution 1. Math

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

    ListNode dHead = new ListNode(0);
    ListNode p = l1;
    ListNode q = l2;
    ListNode cur = dHead;

    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;
        cur.next = new ListNode(sum % 10);
        cur = cur.next;
        if (p != null) p = p.next;
        if (q != null) q = q.next;
    }

    if (carry > 0) {
        cur.next = new ListNode(carry);
    }

    return dHead.next;
}

우리가 두 숫자의 합을 뒤에서부터 계산해 나가는 것 처럼 역순으로 계산하면서 올림값이 발생하면 다음 노드에 하나 추가로 더해줍니다.

Reference

Add Two Numbers - LeetCode