LeetCode 2. Add Two Numbers (两数相加)

题目标签：Linked List, Math

　　题目给了我们两个 Linked List，各代表一个数字，不过顺序的反的。让我们把两个数字相加。

　　和普通的相加其实差不多，只不过变成了 Linked List，还是要用到 / 和 %，具体看code。

Java Solution:

Runtime: 2ms, faster than 87%

Memory Usage: 44MB, less than 85%

完成日期：07/05/2019

关键点：利用 / 取得 carry；利用 % 取得余数

/**
 * 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 cursor1 = l1;
        ListNode cursor2 = l2;
        ListNode cursor3 = dummyHead;
        
        int carry = 0;
        
        while(cursor1 != null || cursor2 != null) // go through both list
        {
            // if node exists, get the value, elsewise, default 0
            int num1 = 0;
            int num2 = 0;
            
            if(cursor1 != null)
                num1 = cursor1.val;
            if(cursor2 != null)
                num2 = cursor2.val;
            
            
            int sum = num1 + num2 + carry;
            
            // update carry and sum
            carry = sum / 10;
            cursor3.next = new ListNode(sum % 10);
            cursor3 = cursor3.next;
            
            // move both list to next node
            if(cursor1 != null)
                cursor1 = cursor1.next;
            if(cursor2 != null)
                cursor2 = cursor2.next;
        }
        
        // at last, still need to check carry for last digit
        if(carry == 1)
            cursor3.next = new ListNode(1);
        
        return dummyHead.next;
    }
}