diff --git a/puzzles/training/tree_iterator.cpp b/puzzles/training/tree_iterator.cpp
new file mode 100644
index 0000000..f56b484
--- /dev/null
+++ b/puzzles/training/tree_iterator.cpp
@@ -0,0 +1,146 @@
+/*
+VIM: let b:cf5build="clang -std=c++20 -O2 -pthread -lstdc++ -I. {SRC} -o {OUT}"
+VIM: let b:cf5run="{OUT}"
+*/
+#include <iostream>
+#include <exception>
+#include <chrono>
+#include <vector>
+
+struct node {
+    node * parent;
+    node * left;
+    node * right;
+    int val;
+    node(int v, node * p)
+        : parent(p)
+        , left(nullptr)
+        , right(nullptr)
+        , val(v)
+    {}
+};
+
+node * build_tree(const std::vector<int>& t){
+    node * root = nullptr;
+    bool left = true;
+    for (int v : t){
+        if (v!=-1){
+            node * n = new node(v, root);
+            if ( root ){
+                if ( left ){
+                    root->left = n;
+                } else {
+                    root->right = n;
+                }
+            }
+            root = n;
+            left = true;
+        } else {
+            if (!root){
+                break;
+            }
+            if (!left){
+                while ( root->parent && !left){
+                    left = root->parent->left == root;
+                    root = root->parent;
+                }
+                if (!left){
+                    break;
+                }
+            }
+            left = false;
+        }
+    }
+    return root;
+}
+
+void print(const node * root){
+    if ( !root ){
+        std::cout << "-1";
+    } else {
+        std::cout << '(';
+        print(root->left);
+        std::cout << ',' << root->val << ',';
+        print(root->right);
+        std::cout << ')';
+    }
+}
+
+
+class tree_itor {
+    node * n;
+public:
+    tree_itor()
+        : n(nullptr)
+    {}
+    tree_itor(node * _n)
+        : n(leftmost(_n))
+    {}
+
+    tree_itor& operator ++ () {
+        if (n->right){
+            n = leftmost(n->right);
+        } else {
+            while (n->parent && n->parent->right==n){
+                n = n->parent;
+            }
+            if (!n->parent){
+                n = nullptr;
+            } else {
+                n = n->parent;
+            }
+        }
+        return *this;
+    }
+
+    bool operator != (const tree_itor& it) const {
+        return n != it.n;
+    }
+
+    int& operator * (){
+        return n->val;
+    }
+private:
+    node * leftmost(node * n){
+        while ( n->left ){
+            n = n->left;
+        }
+        return n;
+    }
+};
+
+int main ( void )
+{try{
+    auto begin = std::chrono::high_resolution_clock::now();
+
+    std::cout << "------------------------" << std::endl;
+    node * tree = build_tree({1, 2, 3, 4, -1, 5, -1, -1, 6, -1, 7, -1, -1, 8, -1, -1, 4, 10, -1, -1, -1 });
+    print(tree);
+    std::cout << std::endl;
+
+    tree_itor it(tree);
+    tree_itor itEnd;
+    for ( ; it != itEnd; ++it ){
+        std::cout << *it << ", ";
+    }
+    std::cout << std::endl;
+    //......
+
+    auto end = std::chrono::high_resolution_clock::now();
+    std::chrono::duration<double> seconds = end - begin;
+    std::cout << "Time: " << seconds.count() << std::endl;
+    return 0;
+}
+catch ( const std::exception& e )
+{
+    std::cerr << std::endl
+            << "std::exception(\"" << e.what() << "\")." << std::endl;
+    return 2;
+}
+catch ( ... )
+{
+    std::cerr  << std::endl
+            << "unknown exception." << std::endl;
+    return 1;
+}}
+