22从上往下打印二叉树

题目描述

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。

Java实现

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

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    public TreeNode(int val) {
        this.val = val;
    }
}

public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        LinkedList<TreeNode> queue = new LinkedList<>();
        ArrayList<Integer> ret = new ArrayList<>();
        queue.addLast(root);
        while (!queue.isEmpty()) {
            int cnt = queue.size();
            while (cnt-- > 0) {
                TreeNode t = queue.removeFirst();
                if (t == null)
                    continue;
                ret.add(t.val);
                queue.addLast(t.left);
                queue.addLast(t.right);
            }
        }
        return ret;
    }
}