diff --git a/problems/3-longest-substring/Makefile b/problems/3-longest-substring/Makefile
new file mode 120000
index 0000000..25eaf27
--- /dev/null
+++ b/problems/3-longest-substring/Makefile
@@ -0,0 +1 @@
+../template/Makefile
\ No newline at end of file
diff --git a/problems/3-longest-substring/lib.c b/problems/3-longest-substring/lib.c
new file mode 100644
index 0000000..07c44e8
--- /dev/null
+++ b/problems/3-longest-substring/lib.c
@@ -0,0 +1,59 @@
+#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
new file mode 100644
index 0000000..2f9b260
--- /dev/null
+++ b/problems/3-longest-substring/lib.h
@@ -0,0 +1,3 @@
+#pragma once
+
+int lengthOfLongestSubstring(char * s);
diff --git a/problems/3-longest-substring/main.c b/problems/3-longest-substring/main.c
new file mode 100644
index 0000000..9651cc0
--- /dev/null
+++ b/problems/3-longest-substring/main.c
@@ -0,0 +1,27 @@
+#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;
+}