题解 | #从上往下打印二叉树#

import java.util.ArrayList;

import java.util.LinkedList;
import java.util.Queue;
/**
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> result = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        if (root != null) {
            queue.add(root);
            while (!queue.isEmpty()) {
                // 取出头上第一个入队元素
                TreeNode treeNode = ((LinkedList<TreeNode>)queue).pollFirst();
                result.add(treeNode.val);
                if (treeNode.left != null) {
                    ((LinkedList<TreeNode>)queue).add(treeNode.left);
                }
                if (treeNode.right != null) {
                    ((LinkedList<TreeNode>)queue).add(treeNode.right);
                }
            }
        }
        return result;
    }
}

解题思想：借助队列先进先出特性，层序遍历