Group data having unique keys and values

Question

I have a following set of numbers:

1  137
1  143
11 37
11 46
11 132
46 65
46 139
69 90

Now, I need to group the data by the first value in a way that no group key is present in the group values. So, for instance, if I were to simply group data, I'd get this result:

1  137
   143
11 37
   46
   132
46 65
   139
69 90

46 here is a group key in the third group and a group value in the second group. In this case I need to merge the group values of the third group into a second group and remove the third group. The end result of the grouping should look like this:

1  137
   143
11 37
   46
   132
   65
   139
69 90

I'm relatively new to C#, so I was wondering if there's a fancy way to do it using LINQ.

Answer 1

Try this LINQ solution:

var numbers = new List<Number>
{
    new Number {X = 1, Y = 137},
    new Number {X = 1, Y = 143},
    new Number {X = 11, Y = 37},
    new Number {X = 11, Y = 46},
    new Number {X = 11, Y = 132},
    new Number {X = 46, Y = 65},
    new Number {X = 46, Y = 139},
    new Number {X = 69, Y = 90}
};
var result = numbers.GroupBy(c => c.X);
var result2 = numbers.FirstOrDefault(c => result.Select(d => d.Key).Contains(c.Y));

var finalResult = numbers.Where(x => x.X == result2?.Y)
                         .Select(x =>  { x.X = result2.X;x.Y = x.Y; return x; } )
                         .Union(numbers.Where(c => c.X != result2?.Y)).GroupBy(c => c.X ,
                         (key, element) => new
                         {
                             Key = key,
                            Element = element.Select(c => c.Y).ToList()
                         });

The result: