RunLength. O(n)

RunLength.cs

using static System.Console;
using System.Linq;

public static class Program {

public static void Main(string[] args)
{
    var input = "wwwwaaadexxxxxxywww";
 
    WriteLine(input);
    
    var counts = 
        input
        .Aggregate(
           (text: "", last: '\0', count: 0),
           (acc, c) =>
               c == acc.last || acc.count == 0
               ? (acc.text, c, acc.count+1)
               : ($"{acc.text} {acc.last}{acc.count}", c, 1)
        )
        .text;
  
 
    Write(counts); //4w 3a 1d 1e 6x 1y 3w

}

    }