adding preorder notes and code

lectures/21_trees_IV/preorder_main.cpp

@@ -0,0 +1,53 @@
+#include <iostream>
+
+class TreeNode {
+public:
+    int val;
+    TreeNode* left;
+    TreeNode* right;
+
+    TreeNode(int value) : val(value), left(NULL), right(NULL) {}
+};
+
+void preorderTraversal(TreeNode* root) {
+      TreeNode *current=root;
+      TreeNode *rightmost;
+      while(current != nullptr){
+        if(current->left != nullptr){
+            rightmost=current->left;
+            while(rightmost->right!=nullptr && rightmost->right!=current){
+                rightmost=rightmost->right;
+            }
+            if(rightmost->right==nullptr){ /* first time */
+                std::cout << current->val << " ";
+                rightmost->right=current;
+                current=current->left;
+            }else{  /* second time */
+                rightmost->right=nullptr;
+                current=current->right;
+            }
+        }else{  /* nodes which do not have left child */
+            std::cout << current->val << " ";
+            current=current->right;
+        }
+    }
+    return;
+}
+
+int main() {
+    TreeNode* root = new TreeNode(1);
+    root->left = new TreeNode(2);
+    root->right = new TreeNode(3);
+    root->left->left = new TreeNode(4);
+    root->left->right = new TreeNode(5);
+    root->left->right->left = new TreeNode(6);
+    root->left->right->right = new TreeNode(7);
+    root->right->right = new TreeNode(8);
+    root->right->right->left = new TreeNode(9);
+
+    std::cout << "Preorder Traversal using Morris Traversal:\n";
+    preorderTraversal(root);
+    std::cout << std::endl;
+
+    return 0;
+}