题目描述

给定一个二叉树，请按照从根节点开始，从上到下、从左到右的顺序（即层序遍历的顺序）打印出每个节点的值。每个节点都是一个包含整数值的节点。

示例：

假设我们有以下二叉树：

    1
   / \
  2   3
 / \
4   5

按照层序遍历的顺序，输出应该是 [1, 2, 3, 4, 5]。

PHP 示例代码

<?php

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

    function __construct($val = 0, $left = null, $right = null) {
        $this->val = $val;
        $this->left = $left;
        $this->right = $right;
    }
}

function levelOrder($root) {
    $result = [];
    if ($root === null) {
        return $result;
    }

    $queue = new SplQueue();
    $queue->enqueue($root);

    while (!$queue->isEmpty()) {
        $levelSize = $queue->count();
        $currentLevel = [];

        for ($i = 0; $i < $levelSize; $i++) {
            $node = $queue->dequeue();
            $currentLevel[] = $node->val;

            if ($node->left !== null) {
                $queue->enqueue($node->left);
            }
            if ($node->right !== null) {
                $queue->enqueue($node->right);
            }
        }

        $result[] = $currentLevel;
    }

    // Flatten the result if needed
    return array_merge(...$result);
}

// 示例用法
$root = new TreeNode(1);
$root->left = new TreeNode(2);
$root->right = new TreeNode(3);
$root->left->left = new TreeNode(4);
$root->left->right = new TreeNode(5);

$result = levelOrder($root);
print_r($result);
?>

Python 示例代码

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def levelOrder(root):
    if not root:
        return []

    result = []
    queue = [root]

    while queue:
        level_size = len(queue)
        current_level = []

        for _ in range(level_size):
            node = queue.pop(0)
            current_level.append(node.val)

            if node.left:
                queue.append(node.left)
            if node.right:
                queue.append(node.right)

        result.extend(current_level)

    return result

# 示例用法
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.left = TreeNode(4)
root.left.right = TreeNode(5)

result = levelOrder(root)
print(result)

JavaScript 示例代码

function TreeNode(val, left, right) {
    this.val = (val===undefined ? 0 : val)
    this.left = (left===undefined ? null : left)
    this.right = (right===undefined ? null : right)
}

function levelOrder(root) {
    if (!root) return [];

    let result = [];
    let queue = [root];

    while (queue.length > 0) {
        let levelSize = queue.length;
        let currentLevel = [];

        for (let i = 0; i < levelSize; i++) {
            let node = queue.shift();
            currentLevel.push(node.val);

            if (node.left) queue.push(node.left);
            if (node.right) queue.push(node.right);
        }

        result = result.concat(currentLevel);
    }

    return result;
}

// 示例用法
let root = new TreeNode(1);
root.left = new TreeNode(2);
root.right = new TreeNode(3);
root.left.left = new TreeNode(4);
root.left.right = new TreeNode(5);

let result = levelOrder(root);
console.log(result);

在以上代码中，我们分别用 PHP、Python 和 JavaScript 实现了二叉树的层序遍历算法。这些示例代码均包含了树的定义、层序遍历函数以及示例用法，以帮助理解和测试算法的正确性。