注：此部落格不再更新，所有最新文章將發表在個人獨立部落格limengting.site。分享技術，記錄生活，歡迎大家關注

題目描述
從上往下打印出二叉樹的每個節點，同層節點從左至右列印。

/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

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

    }

}
*/
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> arrayList = new ArrayList<>();
        if (root == null) return arrayList;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode treeNode = queue.poll();
            arrayList.add(treeNode.val);
            if (treeNode.left != null) {
                queue.offer(treeNode.left);
            }
            if (treeNode.right != null) {
                queue.offer(treeNode.right);
            }
        }
        return arrayList;
    }
}