diff --git a/Longest_Substring.cpp b/Longest_Substring.cpp
new file mode 100644
index 0000000..5ed5941
--- /dev/null
+++ b/Longest_Substring.cpp
@@ -0,0 +1,18 @@
+class Solution {
+public:
+    int lengthOfLongestSubstring(string s) {
+        
+        unordered_map<char,int> index;
+        int start=0,res=0;
+        for(int i=0;i<s.length();i++){
+            
+            if (index.find(s[i]) != index.end() && index[s[i]] >= start)
+                 start = index[s[i]] + 1;
+            
+            index[s[i]] = i;
+            res=max(res,i-start+1);
+        }
+        
+        return res;
+    }
+};
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