diff --git a/Longest_Substring_Without_Repeating_Characters.cpp b/Longest_Substring_Without_Repeating_Characters.cpp
new file mode 100644
index 0000000..65f03f6
--- /dev/null
+++ b/Longest_Substring_Without_Repeating_Characters.cpp
@@ -0,0 +1,33 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+/*
+  Problem: Longest Substring Without Repeating Characters
+  Approach: Sliding Window + Hash Map
+  Time Complexity: O(n)
+  Space Complexity: O(256) ≈ O(1)
+*/
+
+int lengthOfLongestSubstring(string s) {
+    vector<int> lastIndex(256, -1); // store last index of each character
+    int maxLen = 0;
+    int start = 0; // left pointer of window
+
+    for (int end = 0; end < s.length(); end++) {
+        // If character is repeated, move start to right of last occurrence
+        if (lastIndex[s[end]] >= start)
+            start = lastIndex[s[end]] + 1;
+
+        lastIndex[s[end]] = end; // update last index
+        maxLen = max(maxLen, end - start + 1);
+    }
+
+    return maxLen;
+}
+
+int main() {
+    string s = "pwwkew";
+    cout << "Length of longest substring without repeating characters: "
+         << lengthOfLongestSubstring(s) << endl;
+    return 0;
+}