剑指offer之从上往下打印二叉树

思路：

用arraylist模拟一个队列来存储相应的TreeNode

代码：

import java.util.ArrayList;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
/**
思路是用arraylist模拟一个队列来存储相应的TreeNode
*/
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        ArrayList<TreeNode> queue = new ArrayList<TreeNode>();
        if(root == null){
            return list;
        }
        queue.add(root);
        while(queue.size() != 0){
            TreeNode temp = queue.remove(0);
            if(temp.left != null){
                queue.add(temp.left);
            }
            if(temp.right != null){
                queue.add(temp.right);
            }
            list.add(temp.val);
        }
        return list;
    }
}