[LeetCode] 2. Add Two Numbers

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

It is always fun to manipulate linklist.

Iterates, add one by one.

All methods can be optimized with early stop condition (when one linklist reaches null and 0 == carry). 

1. Iterates till end of one linklist, use the second loop to do the rest job.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int carry = 0;
        ListNode preHead = new ListNode(-1);
        ListNode x = preHead;
        
        while(null != l1 && null != l2){
            carry += l1.val+l2.val;
            preHead.next = new ListNode(carry % 10);
            carry /= 10;
            preHead = preHead.next;
            l1 = l1.next;
            l2 = l2.next;
        }
        
        ListNode head = null == l1 ? l2 : l1;
        while( null != head){
            carry += head.val;
            preHead.next = new ListNode(carry % 10);
            carry /= 10;
            preHead = preHead.next;
            head = head.next;
        }
        
        if(0 != carry)
            preHead.next = new ListNode(carry);
            
        return x.next;
    }
}

2. Use one loop, add 0 if link list reaches end.

.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(null == l1)
            return l2;
        if(null == l2)
            return l1;
        
        int carry = 0;
        ListNode pre = new ListNode(0);
        ListNode res = pre;
        while(0 != carry || l1 != null || l2 != null){
            carry += (null == l1 ? 0 : l1.val) + (null == l2 ? 0 : l2.val);
            pre.next = new ListNode(carry % 10);
            carry /= 10;
            pre = pre.next;
            l1 = null == l1 ? l1 : l1.next;
            l2 = null == l2 ? l2 : l2.next;
        }

        return res.next;
        
    }
}

3. Variants, like implement += operation, add value based on l1.

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int carry = 0;
        ListNode res = l1;
        
        ListNode pl1 = new ListNode(0);
        pl1.next = l1;
        while(0 != carry || null != pl1.next || null != l2){
            if(null == pl1.next)
                pl1.next = new ListNode(0);
            carry += pl1.next.val + (null == l2? 0: l2.val);
            
            pl1.next.val = carry % 10;
            carry /= 10;
            pl1 = pl1.next;
            l2 = (null == l2? null : l2.next);
        }
        return l1;
    }
}