diff --git a/3_longest_substring_without_repeating_characters/main.cpp b/3_longest_substring_without_repeating_characters/main.cpp
new file mode 100644
index 0000000..41a6839
--- /dev/null
+++ b/3_longest_substring_without_repeating_characters/main.cpp
@@ -0,0 +1,35 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+class Solution {
+public:
+    int lengthOfLongestSubstring(string s) {
+        int maxLen = 0, currentLen = 0;
+        queue<int> substr;
+        unordered_set<int> charset;
+
+        for (int index = 0; index < s.length(); index++) {
+            char currentChar = s[index];
+
+            // has repeat
+            if (charset.find(currentChar) != charset.end()) {
+                while (substr.front() != currentChar) {
+                    charset.erase(substr.front());
+                    substr.pop();
+                    currentLen--;
+                }
+                charset.erase(currentChar);
+                substr.pop();
+                currentLen--;
+            }
+
+            charset.emplace(currentChar);
+            substr.emplace(currentChar);
+            currentLen++;
+
+            maxLen = max(maxLen, currentLen);
+        }
+
+        return maxLen;
+    }
+};