Longest Substring Without Repeating Characters, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
             String str = "abcxylmzabc"; 
            Console.WriteLine("The input string is " + str); 
            int len = longestUniqueSubsttr(str); 
            Console.Write("The length of the longest non repeating character is " + len);
        }
    static int NO_OF_CHARS = 256;
    public static int longestUniqueSubsttr(String str) 
    { 
        int n = str.Length; 
        // length of current substring 
        int cur_len = 1; 
        // result 
        int max_len = 1; 
        // previous index 
        int prev_index; 
        int i; 
        int[] visited = new int[NO_OF_CHARS]; 
        /* Initialize the visited array as -1, -1 is  used to indicate that character has not been  visited yet. */
        for (i = 0; i < NO_OF_CHARS; i++) { 
            visited[i] = -1; 
        } 
        /* Mark first character as visited by storing the index of first character in visited array. */
        visited[str[0]] = 0; 
  
        /* Start from the second character. First character is already processed (cur_len and max_len are initialized  as 1, and visited[str[0]] is set */
        for (i = 1; i < n; i++) { 
            prev_index = visited[str[i]]; 
            /* If the current character is not present in the already processed substring or it is not part of the current NRCS, then do cur_len++ */
            Console.WriteLine(" i- cur_len > prev_index :   {5} - {0} > {1} ---- [{2}] = {3} ------  prev_index = {4} ", cur_len, prev_index, i, str[i], prev_index, i);
            if (prev_index == -1 || i - cur_len > prev_index) 
                cur_len++; 
            /* If the current character is present in currently considered NRCS, then update NRCS to start from  the next character of the previous instance. */
            else { 
                /* Also, when we are changing the NRCS, we should also check whether the length of the previous NRCS was greater than max_len or not.*/
                if (cur_len > max_len) {
                    max_len = cur_len; 
                }
             cur_len = i - prev_index; 
            } 
            // update the index of current character 
            visited[str[i]] = i; 
        } 
  
        // Compare the length of last NRCS with max_len and update max_len if needed 
        if (cur_len > max_len) 
            max_len = cur_len; 
  
        return max_len; 
    } 
    }
}

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Compilation time: 0,31 sec, absolute running time: 0,17 sec, cpu time: 0,17 sec, average memory usage: 21 Mb, average nr of threads: 4, absolute service time: 0,52 sec