From cdfa46e84c44e54f8a2333153af7bc2804dd0293 Mon Sep 17 00:00:00 2001
From: Vahagn Khachatryan <vahagn.khachatryan@gmail.com>
Date: Sun, 4 Feb 2018 10:40:31 +0000
Subject: [PATCH] preorder tree construction: much better algorithm. No need to
 keep root.

---
 .../training/preorder_tree_construction.cpp   | 48 ++++++++-----------
 1 file changed, 19 insertions(+), 29 deletions(-)

diff --git a/puzzles/interviews/training/preorder_tree_construction.cpp b/puzzles/interviews/training/preorder_tree_construction.cpp
index e2c24ae..e41fcd1 100644
--- a/puzzles/interviews/training/preorder_tree_construction.cpp
+++ b/puzzles/interviews/training/preorder_tree_construction.cpp
@@ -5,17 +5,16 @@ VIM: let b:argv=""
 */
 #include <vector>
 #include <string>
+#include <stack>
 #include <iostream>
 
 struct node {
-    node * root = nullptr;
     node * left = nullptr;
     node * right = nullptr;
     double data = 0;
 
-    node( double d, node * r = nullptr )
-    : root(r)
-    , data(d)
+    node( double d )
+    : data(d)
     {}
 };
 
@@ -26,35 +25,26 @@ node * prefix_to_tree( const std::vector<double>& v ){
     }
 
     node * root = new node(v[0]);
-    node * current = root;
-    node * max_node = nullptr;
+
+    std::stack<node*> stack;
+    stack.push(root);
 
     for ( int i = 1; i < v.size(); ) {
-        if ( v[i] < current->data ){
-            max_node = current;
-            current = new node(v[i++],current);
-            current->root->left = current;
-        } else if ( !max_node
-                    || v[i] < max_node->data ){
-            current = new node(v[i++],current);
-            current->root->right = current;
-        } else if ( current->root ){
-            current = current->root;
-            while ( max_node  
-                    && max_node->data <= current->data ) {
-                node * tmp = max_node;
-                max_node = max_node->root;
-                if ( max_node && tmp == max_node->left ){
-                    break;
-                }
-            }
+        if ( v[i] < stack.top()->data ){
+            auto n = new node(v[i++]);
+            stack.top()->left = n;
+            stack.push(n);
         } else {
-            std::cout << "error" << std::endl;
-            break;
-        }
+            auto c = stack.top();
+            stack.pop();
+            while( !stack.empty() && stack.top()->data < v[i] ){
+                c = stack.top();
+                stack.pop();
+            }
 
-        if ( !current || !root ) {
-            std::cout << "errrrrrror" << std::endl;
+            auto n = new node(v[i++]);
+            c->right = n;
+            stack.push(n);
         }
     }