update merge-sort to reflect API update

sorting.cpp

@@ -42,7 +42,7 @@ vector<int> merge(const vector<int>& left, const vector<int>& right)
     return result;  
 }  
 
-vector<int> sort_custom(vector<int> vec)  
+vector<int> merge_sort(vector<int> vec)  
 {  
     // Termination condition: List is completely sorted if it  
     // only contains a single element.  
@@ -58,9 +58,19 @@ vector<int> sort_custom(vector<int> vec)
     vector<int> right(middle, vec.end());  
   
     // Perform a merge sort on the two smaller vectors  
-    left = sort_custom(left);  
-    right = sort_custom(right);  
+    left = merge_sort(left);  
+    right = merge_sort(right);  
   
     return merge(left, right);  
 }  
 
+vector<int> sort_custom(vector<int> a,int start,int end){
+        vector<int> b;
+	for(int i=0;i<a.size();i++){
+	    if(i>=start || i<=end){
+	        b.push_back(a[i]);
+ 	    }
+        }
+        return merge_sort(b);
+}  
+