# JavaScript: Find longest substring in a given string without repeating characters

## JavaScript Function: Exercise-26 with Solution

Write a JavaScript function to find the longest substring in a given string without repeating characters.

Sample Solution-1:

JavaScript Code:

// Function to find the longest substring without repeating characters
function longest_substring_without_repeating_characters(input) {
// Split the input string into an array of characters
var chars = input.split('');
var curr_char;
var str = "";
var longest_string = "";
var hash = {};

// Loop through each character in the input string
for (var i = 0; i < chars.length; i++) {
curr_char = chars[i];

// Check if the character is not already in the hash table
if (!hash[chars[i]]) {
// Add the character to the current substring
str += curr_char;
// Update the hash table with the character and its index
hash[chars[i]] = {index:i};
} else {
// If there's a repeating character, compare lengths and update if necessary
if (longest_string.length <= str.length) {
longest_string = str;
}
// Get the index of the previous occurrence of the repeating character
var prev_dupeIndex = hash[curr_char].index;
// Extract the substring from the previous occurrence to the current index
var str_FromPrevDupe = input.substring(prev_dupeIndex + 1, i);
// Update the current substring with the new non-repeating characters
str = str_FromPrevDupe + curr_char;
// Reset the hash table for the new substring
hash = {};
// Update the hash table with the characters and their indices in the new substring
for (var j = prev_dupeIndex + 1; j <= i; j++) {
hash[input.charAt(j)] = {index:j};
}
}
}

// Return the longest substring (either the current substring or the previously found longest)
return longest_string.length > str.length ? longest_string : str;
}

// Example usage:
console.log(longest_substring_without_repeating_characters("example.com"));

Output:

gle.com
xample.co

Flowchart:

Live Demo:

See the Pen JavaScript - Find longest substring in a given a string without repeating characters-function-ex- 26 by w3resource (@w3resource) on CodePen.

Sample Solution-2:

JavaScript Code:

// Function to find the longest substring without repeating characters
function longest_substring_without_repeating_characters(input) {
let longestSubstring = "";
let start = 0;
let charIndexMap = {};

for (let end = 0; end < input.length; end++) {
let char = input[end];

if (charIndexMap[char] !== undefined && charIndexMap[char] >= start) {
// If repeating character found, update start index
start = charIndexMap[char] + 1;
}

// Update the index of the current character
charIndexMap[char] = end;

// Update the longest substring
let currentSubstring = input.slice(start, end + 1);
if (currentSubstring.length > longestSubstring.length) {
longestSubstring = currentSubstring;
}
}

return longestSubstring;
}

// Example usage:
console.log(longest_substring_without_repeating_characters("example.com"));

Output:

gle.com
xample.co

Flowchart:

Improve this sample solution and post your code through Disqus.

What is the difficulty level of this exercise?

Test your Programming skills with w3resource's quiz.

﻿