T22.java

package web; /**
 * @program LeetNiu
 * @description: 从上往下打印二叉树
 * @author: mf
 * @create: 2020/01/13 10:23
 */



import java.util.ArrayList;
import java.util.LinkedList;

/**
 * 从上往下打印出二叉树的每个节点，同层节点从左至右打印。
 */
public class T22 {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<>();
        if (root == null) return list;
        // 队列
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            list.add(node.val);
            if (node.left != null) queue.offer(node.left);
            if (node.right != null) queue.offer(node.right);
        }
        return list;
    }
}