摘要：分層數據探索例如無限級分類多級菜單省份城市引言什么是分層數據類似于樹形結構，除了根節點和葉子節點外，所有節點都有一個父節點和一個或多個子節點。接下來我會先通過一般方法和遞歸方法來實現無限極分類，然后再通過兩種數據模型來談一談分層數據的處理。

什么是分層數據？

類似于樹形結構，除了根節點和葉子節點外，所有節點都有一個父節點和一個或多個子節點。

大多數同學都曾在數據庫中處理過分層數據(hierarchical data)，分層數據存在于許多基于數據庫的應用程序中，包括論壇和郵件列表中的分類、商業組織圖表、內容管理系統的分類、產品分類、無限級分類、多級菜單、省份城市等等。但是因為關系數據庫中的表沒有層次關系，只是簡單的平面化的列表；而分層數據具有父－子關系，顯然關系數據庫中的表不能自然地表現出其分層的特性。

接下來我會先通過一般方法和遞歸(recursion)方法來實現無限極分類，然后再通過兩種數據模型來談一談分層數據的處理。

分層數據Hierarchical Data探索(1.遞歸 recursion)

分層數據Hierarchical Data探索(2.鄰接表模型 Adjacency List Model)

分層數據Hierarchical Data探索(3.嵌套集合模型 Nested Set Model)

& 引用賦值

function doloop1(&$i = 1)
{
    print_r($i);
    $i++;
    if ($i <= 10) {
        doloop1($i);
    }
}
doloop1();

static 靜態變量

function doloop2()
{
    static $i = 1;
    print_r($i);
    $i++;
    if ($i <= 10) {
        doloop2();
    }
}

doloop2();

global 全局變量

$i = 1;
function doloop3()
{
    global $i;
    echo $i;
    $i++;
    if ($i <= 10) {
        doloop3();
    }
}
doloop3();

# 模擬數據
       $data = [
            ["id" => 1, "title" => "Electronics", "parent_id" => 0],
            ["id" => 2, "title" => "Laptops & PC", "parent_id" => 1],
            ["id" => 3, "title" => "Laptops", "parent_id" => 2],
            ["id" => 4, "title" => "PC", "parent_id" => 2],
            ["id" => 5, "title" => "Cameras & photo", "parent_id" => 1],
            ["id" => 6, "title" => "Camera", "parent_id" => 5],
            ["id" => 7, "title" => "Phones & Accessories", "parent_id" => 1],
            ["id" => 8, "title" => "Smartphones", "parent_id" => 7],
            ["id" => 9, "title" => "Android", "parent_id" => 8],
            ["id" => 10, "title" => "iOS", "parent_id" => 8],
            ["id" => 11, "title" => "Other Smartphones", "parent_id" => 8],
            ["id" => 12, "title" => "Batteries", "parent_id" => 7],
            ["id" => 13, "title" => "Headsets", "parent_id" => 7],
            ["id" => 14, "title" => "Screen Protectors", "parent_id" => 7],
        ];

/**
 * 值引用獲取無限極分類樹
 * 
 * @param array $data
 * @return array
 */
function make_tree($data)
{  
    $refer = array();  
    $tree = array();  
    foreach($data as $k => $v){  
        $refer[$v["id"]] = & $data[$k];  //創建主鍵的數組引用  
    }  

    foreach($data as $k => $v){  
        $parent_id = $v["parent_id"];   //獲取當前分類的父級id  
        if($parent_id == 0){  
            $tree[] = & $data[$k];   //頂級欄目
        }else{  
            if(isset($refer[$parent_id])){  
                $refer[$parent_id]["children"][] = & $data[$k];  //如果存在父級欄目，則添加進父級欄目的子欄目數組中  
            }  
        }  
    }  

    return $tree;  
}

/**
 * 遞歸獲取無限極分類樹
 * 
 * @param array $data
 * @param int $parent_id
 * @param int $level
 * @return array
 */
function make_tree2($data = [], $parent_id = 0, $level = 0)
{
    $tree = [];
    if ($data && is_array($data)) {
        foreach ($data as $v) {
            if ($v["parent_id"] == $parent_id) {
                $tree[] = [
                    "id" => $v["id"],
                    "level" => $level,
                    "title" => $v["title"],
                    "parent_id" => $v["parent_id"],
                    "children" => make_tree2($data, $v["id"], $level + 1),
                ];
            }
        }
    }
    return $tree;
}

/**
 * 引用賦值方式
 * @param array $data
 * @param int $id
 * @param int $level
 * @return array
 */
function getSubTree($data = [], $id = 0, $level = 0)
{
    static $tree = [];

    foreach ($data as $key => $value) {
        if ($value["parent_id"] == $id) {
            $value["laravel"] = $level;
            $tree[] = $value;
            getSubTree($data, $value["id"], $level + 1);
        }
    }
    return $tree;
}

/**
 * 靜態變量方式
 * @param array $data
 * @param int $id
 * @param int $level
 * @return array
 */
function getSubTree($data = [], $id = 0, $level = 0)
{
    static $tree = [];

    foreach ($data as $key => $value) {
        if ($value["parent_id"] == $id) {
            $value["laravel"] = $level;
            $tree[] = $value;
            getSubTree($data, $value["id"], $level + 1);
        }
    }
    return $tree;
}


/**
 * 全局變量方式
 * @param array $data
 * @param int $id
 * @param int $level
 * @return array
 */

$tree = [];  //先申明變量
function getSubTree($data = [], $id = 0, $level = 0)
{
    global $tree;

    foreach ($data as $key => $value) {
        if ($value["parent_id"] == $id) {
            $value["laravel"] = $level;
            $tree[] = $value;
            getSubTree($data, $value["id"], $level + 1);
        }
    }
    return $tree;
}

/**
 * getParentsByParentId2($categories, 9)
 *
 * @param array $data
 * @param $parent_id
 * @return array
 */
function getParentsByParentId($data = [], $parent_id)
{
    static $categories = [];

    if ($data && is_array($data)) {
        foreach ($data as $item) {
            if ($item["id"] == $parent_id) {
                $categories[] = $item;
                getParentsByParentId($data, $item["parent_id"]);
            }
        }
    }
    return $categories;
}

function getParentsByParentId2($data = [], $parent_id)
{
    $categories = [];

    if ($data && is_array($data)) {
        foreach ($data as $item) {
            if ($item["id"] == $parent_id) {
                $categories[] = $item;
                $categories = array_merge($categories, getParentsByParentId2($data, $item["parent_id"]));
            }
        }
    }
    return $categories;
}