按层打印二叉树

思路：用队列保存二叉树的层节点，不断地从中弹出节点。每一次都用新的temp变量保存当前层的第一个节点

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

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

    }

}
*/
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root == null){
            return res;
        }
        ArrayList<TreeNode> queue = new ArrayList<>();
        queue.add(root);
        while(!queue.isEmpty()){
            TreeNode temp = queue.remove(0);
            if(temp.left != null){
                queue.add(temp.left);
            }
            if(temp.right != null){
                queue.add(temp.right);
            }
            res.add(temp.val);
        }
        return res;
    }
}