diff --git a/18 July  Longest alternating subsequence b/18 July  Longest alternating subsequence
new file mode 100644
index 00000000..e663ba05
--- /dev/null
+++ b/18 July  Longest alternating subsequence	
@@ -0,0 +1,28 @@
+class Solution 
+{
+  public:
+    int alternatingMaxLength(vector<int>& arr) 
+    {
+       
+       if(arr.size()==1)
+       return 1;
+       
+       if(arr.size()==2)
+       {
+           if(arr[0]==arr[1])
+           return 1;
+           return 2;
+       }
+        int ans=1;
+        int pre=arr[0];
+       
+        for(int i=1;i<arr.size()-1;i++)
+       {
+           if(arr[i]>arr[i+1] && arr[i]>pre)
+           ans++,pre=arr[i];
+           else if(arr[i]<pre && arr[i]<arr[i+1])
+           ans++,pre=arr[i];
+       }
+       return ans+1;
+    }
+};