using System.Collections.Immutable;

using System.Linq;

using static System.Console;

​

string seQuence(string items) => items.Aggregate(ImmutableList<(char a, int b)>.Empty

    , (acc, curr) => (acc.Count > 0) switch

    {

        true => acc?[^1].a == curr ? acc.Replace((curr, acc[^1].b), (curr, acc[^1].b + 1))

        : acc.Add((curr, 1)),

        false => acc.Add((curr, 1)),

    }, acc => string.Join("", acc.Select(c => $"{c.a}:{c.b}")));

​

WriteLine(seQuence("aaaabbbcca"));

WriteLine(seQuence("aaabbba"));

WriteLine(seQuence("aaabbccx"));