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

原本以爲很簡單的一道題目，竟然也折騰了一個多小時。

折騰的原因在於對Java對象引用的不熟悉。

<pre name="code" class="java">ListNode L1=new ListNode(1);
ListNode P1=L1;
P1.next=new ListNode(l1.next.val+l2.next.val);
P1=P1.next;

這樣做才能正常地把L1給鏈接起來，而我一開始犯了個非常低級的錯誤就是對P1進行了重新賦值。

ListNode L1=new ListNode(1);
ListNode P1=L1;
P1=new ListNode(l1.next.val+l2.next.val);
P1=P1.next;

這樣犯錯的原因是把C++的指針給誤理解過來了，理解成P1是指針，所以P1改變了後L1也跟着改變。

其實對P1進行重新賦值後，P1就指向了另外一個對象，跟L1就沒關係了。

其他就是問題中的其他沒考慮到的條件。

上代碼

class ListNode {
	int val;
	ListNode next;

	ListNode(int x) {
		val = x;
		next = null;
	}
}

public class Solution {
	

	public static void main(String[] args) {
		ListNode l1 = new ListNode(0), l2 = new ListNode(0);
		ListNode P1 = l1, P2 = l2;

		// l1,P2=l2;
		int[] l1val = { 1, 2 };// , 9, 9, 9, 9, 8, 9, 9, 9 };
		int[] l2val = { 3, 5 };// , 2 };
		for (int indexL1val = 0; indexL1val < l1val.length; ++indexL1val) {
			P1.val = l1val[indexL1val];
			P1.next = new ListNode(0);
			P1 = P1.next;
		}

		for (int indexL2val = 0; indexL2val < l2val.length; ++indexL2val) {
			P2.val = l2val[indexL2val];
			P2.next = new ListNode(0);
			P2 = P2.next;
		}
		ListNode resultListNode = new Solution().addTwoNumbers(l1, l2);
		for (; resultListNode != null; resultListNode = resultListNode.next)
			System.out.println(resultListNode.val);

	}

	private ListNode addTwoNumbers(ListNode l1, ListNode l2) {

		int isBigTen = 0;
		ListNode L = new ListNode(0);

		ListNode P = L;
		ListNode l1tmp = l1, l2tmp = l2;
		for (; l1tmp != null && l2tmp != null; l1tmp = l1tmp.next, l2tmp = l2tmp.next) {
			P.val = (l1tmp.val + l2tmp.val + isBigTen);
			isBigTen = 0;// 記得消去
			if (P.val >= 10) {
				P.val = P.val - 10;
				isBigTen = 1;
			}
			if (l1tmp.next != null && l2tmp.next != null) {
				P.next = new ListNode(0);
				P = P.next;
			}
		}
		while(l1tmp!=null){
			P.next=new ListNode(l1tmp.val+isBigTen);
			isBigTen=0;
			P=P.next;
			if(P.val>=10){
			    P.val-=10;
			    isBigTen=1;
			}
			l1tmp=l1tmp.next;
		}
		while(l2tmp!=null){
			P.next=new ListNode(l2tmp.val+isBigTen);
			isBigTen=0;
			P=P.next;
			if(P.val>=10){
			    P.val-=10;
			    isBigTen=1;
			}
			l2tmp=l2tmp.next;
		}
		if(isBigTen==1)
		{
			P.next=new ListNode(1);
		}
		return L;

	}
}