【剑指offer】从上往下打印二叉树

宽度优先搜索BFS，不懂得小伙伴百度一下BFS...

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

public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.remove();
            if (node == null) {
                continue;
            }
            list.add(node.val);
            queue.add(node.left);
            queue.add(node.right);
        }
        return list;
    }
}