diff --git a/lengthOfLongestSubstring.py b/lengthOfLongestSubstring.py
new file mode 100644
index 0000000..d30e840
--- /dev/null
+++ b/lengthOfLongestSubstring.py
@@ -0,0 +1,24 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+class Solution {
+public:
+    int lengthOfLongestSubstring(string s) {
+        vector<int> index(256, -1); // stores last index of each character
+        int maxLen = 0;
+        int start = 0; // left boundary of current window
+
+        for (int end = 0; end < s.size(); end++) {
+            char c = s[end];
+
+            // If character was seen before and is inside the current window
+            if (index[c] >= start)
+                start = index[c] + 1;
+
+            index[c] = end; // update last index of this char
+            maxLen = max(maxLen, end - start + 1);
+        }
+
+        return maxLen;
+    }
+};