diff --git a/problems/3-longest-substring/Makefile b/problems/3-longest-substring/Makefile
deleted file mode 120000
index 25eaf27..0000000
--- a/problems/3-longest-substring/Makefile
+++ /dev/null
@@ -1 +0,0 @@
-../template/Makefile
\ No newline at end of file
diff --git a/problems/3-longest-substring/lib.c b/problems/3-longest-substring/lib.c
deleted file mode 100644
index 07c44e8..0000000
--- a/problems/3-longest-substring/lib.c
+++ /dev/null
@@ -1,59 +0,0 @@
-#include <string.h>
-#include "lib.h"
-
-void resetArray(int* count) {
-    int length = '~' - 32 + 1;
-    for (int i=0; i<length; i++) {
-        count[i] = 0;
-    }
-}
-
-int lengthOfLongestSubstring(char * s) {
-    // Create an array to count every symbol
-    // '~' is one of the last symbols in ascii table (except DEL)
-    int count['~' - 32 + 1];
-    resetArray(count);
-    
-    int len = strlen(s);
-    // Start of the current sequence
-    int start = 0;
-    // Result will be stored here
-    int maxlen = 0;
-
-    while (start < len) {
-        // End of the current sequence
-        int end = start;
-
-        while (end < len) {
-            // Calculate index in array
-            int ch = s[end] - 32;
-            // Symbol repeated
-            if (count[ch] == 1) {
-                // Fill array with 0
-                resetArray(count);
-                // Save max length
-                int newmaxlen = end - start;
-                if (newmaxlen > maxlen)
-                    maxlen = newmaxlen;
-                // Move to the next sequence
-                start++;
-                end = start;
-            }
-            else {
-                // Save the symbol
-                count[ch]++;
-                // Move to the next symbol
-                end++;
-            }
-        }
-        // Save max length
-        int newmaxlen = end - start;
-        if (newmaxlen > maxlen)
-            maxlen = newmaxlen;
-        // Move to the next sequence
-        start++;
-        end = start;
-    }
-
-    return maxlen;
-}
diff --git a/problems/3-longest-substring/lib.h b/problems/3-longest-substring/lib.h
deleted file mode 100644
index 2f9b260..0000000
--- a/problems/3-longest-substring/lib.h
+++ /dev/null
@@ -1,3 +0,0 @@
-#pragma once
-
-int lengthOfLongestSubstring(char * s);
diff --git a/problems/3-longest-substring/main.c b/problems/3-longest-substring/main.c
deleted file mode 100644
index 9651cc0..0000000
--- a/problems/3-longest-substring/main.c
+++ /dev/null
@@ -1,27 +0,0 @@
-#include <stdio.h>
-#include "lib.h"
-
-int main() {
-    printf("Space: %d, z: %d\n", ' ', 'z');
-    char* s1 = "abcabcbb";
-    int res1 = lengthOfLongestSubstring(s1);
-    printf("Test 1: %d\n", res1 == 3);
-
-    char* s2 = "bbbbb";
-    int res2 = lengthOfLongestSubstring(s2);
-    printf("Test 2: %d\n", res2 == 1);
-
-    char* s3 = "pwwkew";
-    int res3 = lengthOfLongestSubstring(s3);
-    printf("Test 3: %d\n", res3 == 3);
-    
-    char* s4 = "";
-    int res4 = lengthOfLongestSubstring(s4);
-    printf("Test 4: %d\n", res4 == 0);
-
-    char* s5 = "a";
-    int res5 = lengthOfLongestSubstring(s5);
-    printf("Test 5: %d\n", res5 == 1);
-
-    return 0;
-}
diff --git a/problems/3-longest-substring/main.rb b/problems/3-longest-substring/main.rb
new file mode 100644
index 0000000..7193f1e
--- /dev/null
+++ b/problems/3-longest-substring/main.rb
@@ -0,0 +1,44 @@
+# @param {String} s
+# @return {Integer}
+def length_of_longest_substring(s)
+  # Create a hash map to save
+  # the position of every symbol
+  indexes = Hash.new(-1)
+
+  # Start and end of the current sequence s[i...j]
+  i = 0
+  j = 0
+
+  # Result will be stored here
+  maxlen = 0
+
+  while i < s.length
+    # While we haven't reached the end of the string
+    # and haven't seen current character before
+    while j < s.length && indexes[s[j]] == -1
+      # Save character's position
+      indexes[s[j]] = j
+      # Move to the next character
+      j += 1
+    end
+
+    # We just reached the end of the string
+    # and we will never find a longer substring.
+    return [maxlen, j - i].max if j == s.length
+
+    # It's not the end of the string,
+    # repeated character found.
+    # Save max length and set "i"
+    # to the character right after the first repeated.
+    maxlen = [maxlen, j - i].max
+    new_i = indexes[s[j]] + 1
+    while i != new_i
+      indexes[s[i]] = -1
+      i += 1
+    end
+    indexes[s[j]] = j
+    j += 1
+  end
+
+  maxlen
+end
diff --git a/problems/3-longest-substring/test.rb b/problems/3-longest-substring/test.rb
new file mode 100644
index 0000000..b66df31
--- /dev/null
+++ b/problems/3-longest-substring/test.rb
@@ -0,0 +1,26 @@
+require_relative 'main'
+require 'test/unit'
+
+class TestLengthOfLongestSubstring < Test::Unit::TestCase
+  def test_regular_string
+    assert_equal(3, length_of_longest_substring('abcabcbb'))
+    assert_equal(3, length_of_longest_substring('pwwkew'))
+    assert_equal(2, length_of_longest_substring('abba'))
+  end
+
+  def test_empty_string
+    assert_equal(0, length_of_longest_substring(''))
+  end
+
+  def test_one_character
+    assert_equal(1, length_of_longest_substring('a'))
+  end
+
+  def test_repeating_characters
+    assert_equal(1, length_of_longest_substring('bbbbbb'))
+  end
+
+  def test_very_long_string
+    assert_equal(86, length_of_longest_substring('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!\"#&()*+/<=>?@[]^`{|}~ ' * 1000))
+  end
+end