《剑指Offer》-32.从上到下打印二叉树

题干

不分行从上到下打印二叉树

从上到下打印二叉树到每个节点，同一层到节点按照从左到右到顺序打印。例如，输入下图到二叉树，则依次打印出8，6，10，5，7，9，11.二叉树节点到定义如下：

class TreeNode{
    int val;
    TreeNode left;
    TreeNode right;
}

graph TD
8-->6
8-->10
6-->5
6-->7
10-->9
10-->11

解题思路

每次打印一个节点的时候，如果该节点有子节点，则把该节点的子节点放到一个队列的末尾。接下来到队列到头部取出最早进入队列到节点，重复前面到打印操作，直至队列中所有到节点都被打印处理。

代码实现

<?php

class TreeNode
{
    private $val;
    private $left;
    private $right;

    public function __set($name, $value)
    {
        $this->$name = $value;
    }

    public function __get($name)
    {
        return $this->$name;
    }
}

function getTree()
{
    $node1 = new TreeNode();
    $node1->val = 8;
    $node2 = new TreeNode();
    $node2->val = 6;
    $node3 = new TreeNode();
    $node3->val = 10;
    $node1->left = $node2;
    $node1->right = $node3;
    $node4 = new TreeNode();
    $node4->val = 5;
    $node5 = new TreeNode();
    $node5->val = 7;
    $node2->left = $node4;
    $node2->right = $node5;
    $node6 = new TreeNode();
    $node6->val = 9;
    $node7 = new TreeNode();
    $node7->val = 11;
    $node3->left = $node6;
    $node3->right = $node7;

    return $node1;
}

function printFromTopToBottom($tree)
{
    if ($tree == null) {
        return;
    }

    $queue = [];
    array_push($queue, $tree);
    while (!empty($queue)) {
        $node = array_shift($queue);
        echo $node->val.' ';

        if ($node->left != null) {
            array_push($queue, $node->left);
        }

        if ($node->right != null) {
            array_push($queue, $node->right);
        }
    }
}


$tree = getTree();
printFromTopToBottom($tree);