Two Lists Sum.java

M
tags: Linked List

给两个Linked list, sum up and 合成新的list

```

/*
You have two numbers represented by a linked list, where each node contains a single digit.
The digits are stored in reverse order, such that the 1’s digit is at the head of the list.
Write a function that adds the two numbers and returns the sum as a linked list.

Example
Given two lists, 3->1->5->null and 5->9->2->null, return 8->0->8->null

Tags Expand 
Linked List Backtracking
//TODO: check 9chapter solution
Thinking process:
Simply add 2 lists’ values together.
Handle the carrier
Use dummy node at beginning.
*/

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;      
 *     }
 * }
 */
public class Solution {
    /**
     * @param l1: the first list
     * @param l2: the second list
     * @return: the sum list of l1 and l2 
     */
    public ListNode addLists(ListNode l1, ListNode l2) {
        ListNode rst = new ListNode(0);
        ListNode dummy = rst;
        int carrier = 0;
        //while
        while (l1 != null || l2 != null) {
            if (l1 != null) {
                carrier += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                carrier += l2.val;
                l2 = l2.next;
            }
            rst.next = new ListNode(carrier % 10);
            carrier = carrier / 10;
            rst = rst.next;
        }
        //check the carrier
        if (carrier == 1) {
            rst.next = new ListNode(1);
        }
        return dummy.next;
    }
}


```