002_AddTwoNumbers

/***
 * 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 contain a single digit.
 * Add the two numbers and return it as a linked list.
 *
 * You may assume the two numbers do not contain any leading zero, except the number 0 itself.
 *
 * Example:
 *
 * Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
 * Output: 7 -> 0 -> 8
 * Explanation: 342 + 465 = 807.
 *
 */

/**
 * Definition for singly-linked list.
 */
public class ListNode {
    public int val;
    public ListNode next;

    public ListNode(int x) {
        val = x;
    }
}

 

public class N002_AddTwoNumbers {

    public static ListNode add(ListNode l1, ListNode l2) {

        /***
         * 多餘的步驟！
        if (l1 == null) {
            return l2;
        }

        if (l2 == null) {
            return l1;
        }
         ***/

        ListNode result = new ListNode(-1);        //關鍵步驟，要多一個頭節點用來返回結果
        ListNode l3 = result;
        int value = 0;

        while (l1 != null || l2 != null) {
            if (l1 != null) {
                value = value + l1.val;
                l1 = l1.next;
            }

            if (l2 != null) {
                value = value + l2.val;
                l2 = l2.next;
            }

            l3.next = new ListNode(value % 10);
            l3 = l3.next;
            value = value / 10;
        }

        /***
         * 最後一步的進位檢測，很容易漏！！！
         */
        if(value==1)
            l3.next=new ListNode(1);


        return result.next;
    }

    public static void main(String args[]) {

        ListNode l1tmp2 = new ListNode(8);
        ListNode l1tmp1 = new ListNode(5);
        l1tmp1.next = l1tmp2;
        ListNode l1 = new ListNode(3);
        l1.next = l1tmp1;
        printListNode(l1);

        ListNode l2tmp2 = new ListNode(2);
        ListNode l2tmp1 = new ListNode(8);
        l2tmp1.next = l2tmp2;
        ListNode l2 = new ListNode(7);
        l2.next = l2tmp1;
        printListNode(l2);

        ListNode result = add(l1, l2);
        printListNode(result);
    }


    private static void printListNode(ListNode ln) {
        String out = "";
        ListNode tmp = ln;
        out = tmp.val + out;
        while (tmp.next != null) {
            tmp = tmp.next;
            out = tmp.val + out;
        }
        System.out.println(out);
    }
}