注意java里的“引用”

import java.util.ArrayList;

class TreeNode{
	int val;
	TreeNode left;
	TreeNode right;
	TreeNode(int x)	{
		val = x;
		left = null;
		right = null;
	}
}
public class Solution {
	public static void main(String args[]){
		TreeNode n1 = new TreeNode(1);
		TreeNode n2 = new TreeNode(2);
		TreeNode n3 = new TreeNode(3);
		n1.left = n2;
		n1.right = n3;
		ArrayList<TreeNode> list = new ArrayList<TreeNode>();
		list.add(n1);
		TreeNode n4 = new TreeNode(4);
		n1.right = n4;
		list.add(n1);
		System.out.println(list.size());
		for(int i = 0; i < list.size(); i++){
			System.out.println(list.get(i).val);
			System.out.println(list.get(i).left.val);
			System.out.println(list.get(i).right.val);
		}
	}
}

该段代码的输出是2
1
2
4
1
2

4

也就是说，把一个结点加入到arraylist之后，再来修改这个结点，arraylist里本来的值也会改变，当初传进去的其实是结点的引用。