diff --git a/include/s0003_longest_substring_without_repeating_characters.hpp b/include/s0003_longest_substring_without_repeating_characters.hpp
new file mode 100644
index 0000000..e1f6f9b
--- /dev/null
+++ b/include/s0003_longest_substring_without_repeating_characters.hpp
@@ -0,0 +1,21 @@
+#ifndef S0003_LONGEST_SUBSTRING_WITHOUT_REPEATING_CHARACTERS_HPP
+#define S0003_LONGEST_SUBSTRING_WITHOUT_REPEATING_CHARACTERS_HPP
+
+#include <map>
+#include <string>
+
+class Solution {
+ public:
+  /**
+   * @brief Longest Substring Without Repeating Characters
+   *
+   * Given a string `s`, find the length of the **longest substring** without
+   * repeating characters.
+   *
+   * @param s the given string
+   * @return the longest substring without repeating characters
+   */
+  int lengthOfLongestSubstring(std::string s);
+};
+
+#endif
diff --git a/src/s0003_longest_substring_without_repeating_characters.cpp b/src/s0003_longest_substring_without_repeating_characters.cpp
new file mode 100644
index 0000000..b8e8a9b
--- /dev/null
+++ b/src/s0003_longest_substring_without_repeating_characters.cpp
@@ -0,0 +1,26 @@
+#include "s0003_longest_substring_without_repeating_characters.hpp"
+
+int Solution::lengthOfLongestSubstring(std::string s) {
+  // index_forward points to the frontmost element
+  // index_backward points to the last repeated element
+  //
+  // for
+  //    check if index_forward pointed element exists in hash map
+  //            if exists, index_backward = max(index_backward, the index of
+  //            this element)
+  //    push
+  //    index_forward += 1
+  //    len = max(index_forward, index_backward)
+  std::map<char, int> map;
+  int index_backward{-1};
+  int index_forward{0};
+  int len{0};
+  for (; index_forward < s.length(); index_forward++) {
+    if (map.count(s[index_forward]) == 1) {
+      index_backward = std::max(index_backward, map.at(s[index_forward]));
+    }
+    map[s[index_forward]] = index_forward;
+    len = std::max(len, index_forward - index_backward);
+  }
+  return len;
+}
diff --git a/tests/s0003_longest_substring_without_repeating_characters.cpp b/tests/s0003_longest_substring_without_repeating_characters.cpp
new file mode 100644
index 0000000..cfbe224
--- /dev/null
+++ b/tests/s0003_longest_substring_without_repeating_characters.cpp
@@ -0,0 +1,21 @@
+#include "s0003_longest_substring_without_repeating_characters.hpp"
+
+#include <gtest/gtest.h>
+
+TEST(Problem3, Case1) {
+  std::string s{std::string("abcabcbb")};
+  Solution solution;
+  EXPECT_EQ(solution.lengthOfLongestSubstring(s), 3);
+}
+
+TEST(Problem3, Case2) {
+  std::string s{std::string("bbbbb")};
+  Solution solution;
+  EXPECT_EQ(solution.lengthOfLongestSubstring(s), 1);
+}
+
+TEST(Problem3, Case3) {
+  std::string s{std::string("pwwkew")};
+  Solution solution;
+  EXPECT_EQ(solution.lengthOfLongestSubstring(s), 3);
+}