Power Query code to find parent

Anonymous 

Ok. You can just keep the first item in the list then:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUQpILSrOz1MwVIrViVYyQggYgQWMEQLG6AIm6AKmYAEThIAZuoA5uhYLdAFLdHcYGqArMQS6NRYA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Level = _t, Name = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Level", Int64.Type}, {"Name", type text}}),

    #"Added Custom" = Table.AddColumn(#"Changed Type", "Parent", each try Table.SelectRows(#"Changed Type", (inner)=>inner[Level]=[Level]-1)[Name]{0} otherwise null, type text)
in
    #"Added Custom"

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Anonymous 

Then we need to add an index to establish that order:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUQpILSrOz1MwVIrViVYyQggYgQWMEQLG6AIm6AKmYAEThIAZuoA5uhYLdAFLdHcYGqArMQS6NRYA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Level = _t, Name = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Level", Int64.Type}, {"Name", type text}}),

    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 1, 1, Int64.Type),
    #"Added Custom" = Table.AddColumn(#"Added Index", "Parent", each try List.Last(Table.SelectRows(#"Added Index", (inner)=>inner[Level]=[Level]-1 and inner[Index]<[Index])[Name]) otherwise null, type text),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Index"})
in
    #"Removed Columns"

It would have helped if you had shown the expected result from the beginning

Please mark the question solved when done and consider giving a thumbs up if posts are helpful.

Contact me privately for support with any larger-scale BI needs, tutoring, etc.

Cheers