Which is a better data structure using mongodb?

I'm still confused which structure is better structure using MongoDB.

Place child in parent:

{
    "_id": "528ba7691738025d11aab772",
    "post": "Hello 1",
    "childs": [
        {
            "_id": "528ba7691738025d11aab555",
            "post": "Hello 2",
            "childs": []
        },
        {
            "_id": "528ba7691738025d117631783",
            "post": "Hello 22",
            "childs": [
                {
                    "_id": "528ba7612D38025d11aab772",
                    "post": "Hello 3",
                    "childs": []
                }
            ]
        }
    ]
}

Place child equal with the parent like relational database do:

[
    {
        "_id": "528ba7691738025d11aab772",
        "post": "Hello 1",
        "parent_id": null
    },
    {
        "_id": "528ba7691738025d11aab555",
        "post": "Hello 2",
        "parent_id": "528ba7691738025d11aab772"
    },
    {
        "_id": "528ba7691738025d117631783",
        "post": "Hello 22",
        "parent_id": "528ba7691738025d11aab772"
    },
    {
        "_id": "528ba7612D38025d11aab772",
        "post": "Hello 3",
        "parent_id": "528ba7691738025d117631783"
    }
]