diff --git a/leetcode/problem_3/sol1.c b/leetcode/problem_3/sol1.c
index 2acd0823..3d2efa7d 100644
--- a/leetcode/problem_3/sol1.c
+++ b/leetcode/problem_3/sol1.c
@@ -1,12 +1,33 @@
-int lengthOfLongestSubstring(char *str)
+/**
+ * \file
+ * \brief [3. Longest
+ * substring](https://leetcode.com/problems/longest-substring-without-repeating-characters/)
+ * optimized solution
+ * \details Given a string `s`, find the length of the longest substring without
+ * repeating characters.
+ */
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/**
+ * @brief Optimized Longest substring function.
+ *
+ * @param str NULL terminated alpha-numeric string to search
+ * @return length of longest substring
+ * @note no spaces or other non-alphanumeric characters
+ */
+int lengthOfLongestSubstring(const char *str)
 {
-    int n = strlen(str);
+    size_t n = strlen(str);
 
     if (!n)
         return 0;
 
-    int L_len = 1;  // lenght of longest substring
-    int C_len = 1;  // lenght of current substring
+    int L_len = 1;  // length of longest substring
+    int C_len = 1;  // length of current substring
 
     int P_ind, i;      //  P_ind for previous index
     int visited[256];  //  visited will keep track of visiting char for the last
@@ -33,33 +54,25 @@ int lengthOfLongestSubstring(char *str)
         L_len = C_len;
     return L_len;
 }
-/* Brute force */
-int lengthOfLongestSubstring(char *s)
-{
-    int cur_max = 0, max = 0;
-    int counter[255];
-    int end = 0;
 
-    memset(counter, 0, sizeof(int) * 255);
-    while (end < strlen(s))
-    {
-        if (counter[s[end]] == 0)
-        {
-            counter[s[end]]++;
-            end++;
-            cur_max++;
-        }
-        else
-        {
-            char c = s[end];
-            memset(counter, 0, 255 * sizeof(int));
-            if (cur_max >= max)
-                max = cur_max;
-            cur_max = 0;
-            while (s[end - 1] != c) end--;
-        }
-    }
-    if (cur_max >= max)
-        max = cur_max;
-    return max;
+/**
+ * @brief Main function
+ * @return 0
+ */
+int main()
+{
+    int test1 = lengthOfLongestSubstring("abcabcbb");
+    assert(test1 == 3);
+    printf("Test 1 passed\n");
+    int test2 = lengthOfLongestSubstring("bbbbb");
+    assert(test2 == 1);
+    printf("Test 2 passed\n");
+    int test3 = lengthOfLongestSubstring("pwwkew");
+    assert(test3 == 3);
+    printf("Test 3 passed\n");
+    int test4 = lengthOfLongestSubstring("");
+    assert(test4 == 0);
+    printf("Test 4 passed\n");
+
+    return 0;
 }
diff --git a/leetcode/problem_3/sol2.c b/leetcode/problem_3/sol2.c
new file mode 100644
index 00000000..64f18246
--- /dev/null
+++ b/leetcode/problem_3/sol2.c
@@ -0,0 +1,72 @@
+/**
+ * \file
+ * \brief [3. Longest
+ * substring](https://leetcode.com/problems/longest-substring-without-repeating-characters/)
+ *  brute force solution
+ * \details Given a string `s`, find the length of the longest substring without
+ * repeating characters.
+ */
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/**
+ * @brief Optimized Longest substring function.
+ *
+ * @param str NULL terminated alpha-numeric string to search
+ * @return length of longest substring
+ * @note no spaces or other non-alphanumeric characters
+ */
+int lengthOfLongestSubstring(const char *s)
+{
+    int cur_max = 0, max = 0;
+    int counter[255];
+    int end = 0;
+
+    memset(counter, 0, sizeof(int) * 255);
+    while (end < strlen(s))
+    {
+        if (counter[s[end]] == 0)
+        {
+            counter[s[end]]++;
+            end++;
+            cur_max++;
+        }
+        else
+        {
+            char c = s[end];
+            memset(counter, 0, 255 * sizeof(int));
+            if (cur_max >= max)
+                max = cur_max;
+            cur_max = 0;
+            while (s[end - 1] != c) end--;
+        }
+    }
+    if (cur_max >= max)
+        max = cur_max;
+    return max;
+}
+
+/**
+ * @brief Main function
+ * @return 0
+ */
+int main()
+{
+    int test1 = lengthOfLongestSubstring("abcabcbb");
+    assert(test1 == 3);
+    printf("Test 1 passed\n");
+    int test2 = lengthOfLongestSubstring("bbbbb");
+    assert(test2 == 1);
+    printf("Test 2 passed\n");
+    int test3 = lengthOfLongestSubstring("pwwkew");
+    assert(test3 == 3);
+    printf("Test 3 passed\n");
+    int test4 = lengthOfLongestSubstring("");
+    assert(test4 == 0);
+    printf("Test 4 passed\n");
+
+    return 0;
+}