C Exercises: No repeating characters in a substring

Last update on August 09 2025 12:47:08 (UTC/GMT +8 hours)

2. Longest Substring Without Repeating Characters Variants

Write a C program to find the length of the longest substring of a given string without repeating characters.

Note:
1) Given string consists of English letters, digits, symbols and spaces.
2) 0 <= Given string length <= 5 * 104

Difficulty: Medium.
Company: Amazon, Google, Bloomberg, Microsoft, Adobe, Apple, Oracle, Facebook and more.

Input String : xyzxyzyy
The longest Substring Length : 3
The longest substring : xyz

Input String : pickoutthelongestsubstring
The longest Substring Length : 8
The longest substring : ubstring

Input String : ppppppppppppp
The longest substring : p
The longest Substring Length : 1

Input String : Microsoft
The longest substring : Micros
The longest Substring Length : 6

C Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
static int length_of_longest_substring(char *str1)
{
   int offset[128];
   int substr_max_len = 0;
   int len = 0;
   int index = 0;
   memset(offset, 0xff, sizeof(offset));
   while (*str1 != '\0') {
       if (offset[*str1] == -1) {
           len++;
         }
                   else {
           if (index - offset[*str1] > len)
                                     {
               len++;
           }
                            else
                            {
                len = index - offset[*str1];
           }
       }
       if (len > substr_max_len)
                  {
           substr_max_len = len;
       }
       offset[*str1++] = index++;
   }
   return substr_max_len;
}
int main(void)
 {
   char *str1 = "xyzxyzyy";
         printf("\nOriginal String: %s", str1);
         printf("\nLength of the longest substring without repeating characters: %d\n", length_of_longest_substring(str1));
         return 0;
}

Sample Output:

Original String: xyzxyzyy
Length of the longest substring without repeating characters: 3

Pictorial Presentation:

C Programming: Find length of the longest substring of a given string without repeating characters.

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to find the longest substring without repeating characters using a sliding window approach.
Write a C program to return the starting index of the longest substring without repeating characters in a given string.
Write a C program to count the number of longest substrings without repeating characters if there are multiple.
Write a C program to find the longest substring without repeating vowels in a given string.

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