Create finding catalan number

Author: gs146

finding catalan number

@@ -0,0 +1,18 @@
+long long int catalan(int n, int k)
+    {
+        long long int res=1;
+        if(k > n-k)
+            k = n-k;
+        
+        for(int i=0;i<k;i++)
+        {
+            res*=(n-i);
+            res/=(i+1);
+        }
+        return res;
+    }
+    int numTrees(int n)
+    {
+        unsigned long int c = catalan(2*n,n);
+        return (c/(n+1));
+    }