diff --git a/src/main/java/com/github/pedrovgs/problem81/optimal_binary_search_tree.java b/src/main/java/com/github/pedrovgs/problem81/optimal_binary_search_tree.java
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+
+public class optimal_binary_search_tree 
+{ 
+    // A recursive function to calculate cost of 
+        // optimal binary search tree 
+    static int optCost(int freq[], int i, int j) 
+    { 
+       // Base cases 
+       if (j < i)      // no elements in this subarray 
+         return 0; 
+       if (j == i)     // one element in this subarray 
+         return freq[i]; 
+       
+       // Get sum of freq[i], freq[i+1], ... freq[j] 
+       int fsum = sum(freq, i, j); 
+       
+       // Initialize minimum value 
+       int min = Integer.MAX_VALUE; 
+       
+       // One by one consider all elements as root and  
+           // recursively find cost of the BST, compare the  
+           // cost with min and update min if needed 
+       for (int r = i; r <= j; ++r) 
+       { 
+           int cost = optCost(freq, i, r-1) +  
+                          optCost(freq, r+1, j); 
+           if (cost < min) 
+              min = cost; 
+       } 
+       
+       // Return minimum value 
+       return min + fsum; 
+    } 
+      
+    // The main function that calculates minimum cost of 
+        // a Binary Search Tree. It mainly uses optCost() to 
+        // find the optimal cost. 
+    static int optimalSearchTree(int keys[], int freq[], int n) 
+    { 
+         // Here array keys[] is assumed to be sorted in  
+             // increasing order. If keys[] is not sorted, then 
+             // add code to sort keys, and rearrange freq[]  
+             // accordingly. 
+         return optCost(freq, 0, n-1); 
+    } 
+      
+    // A utility function to get sum of array elements  
+        // freq[i] to freq[j] 
+    static int sum(int freq[], int i, int j) 
+    { 
+        int s = 0; 
+        for (int k = i; k <=j; k++) 
+           s += freq[k]; 
+        return s; 
+    } 
+      
+    // Driver code 
+    public static void main(String[] args) { 
+        int keys[] = {10, 12, 20}; 
+        int freq[] = {34, 8, 50}; 
+        int n = keys.length; 
+        System.out.println("Cost of Optimal BST is " + 
+                         optimalSearchTree(keys, freq, n)); 
+    } 
+} 
+