diff --git a/puzzles/interviews/training/preorder_tree_construction.cpp b/puzzles/interviews/training/preorder_tree_construction.cpp
index 7548431..0c6c347 100644
--- a/puzzles/interviews/training/preorder_tree_construction.cpp
+++ b/puzzles/interviews/training/preorder_tree_construction.cpp
@@ -1,4 +1,5 @@
 #include <vector>
+#include <string>
 #include <iostream>
 
 struct node {
@@ -39,7 +40,7 @@ node * prefix_to_tree( const std::vector<double>& v ){
                 node * tmp = max_node;
                 max_node = max_node->root;
                 if ( max_node && tmp == max_node->left ){
-                    break;                    
+                    break;
                 }
             }
         } else {
@@ -99,4 +100,4 @@ int main( void ) {
     test( {8, 4, 2, 1,    6,       12, 10,    11, 14,     15} );
 
     return 0;
-}
\ No newline at end of file
+}