adding the iterative approach using stacks

lectures/21_trees_IV/preorder_iterative.cpp

@@ -0,0 +1,45 @@
+#include <iostream>
+#include <stack>
+
+class TreeNode {
+public:
+    int val;
+    TreeNode* left;
+    TreeNode* right;
+    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+};
+
+// pre order traverse a binary tree, iteratively.
+void preorderTraversal(TreeNode* root) {
+    if (root == nullptr) return;
+
+    std::stack<TreeNode*> st;
+    st.push(root);
+
+    while (!st.empty()) {
+        TreeNode* current = st.top();
+        st.pop();
+        std::cout << current->val << " ";
+
+        if (current->right) st.push(current->right);  // push right first
+        if (current->left) st.push(current->left);    // then push left
+    }
+}
+
+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 << "Inorder Traversal: ";
+    preorderTraversal(root);
+    std::cout << std::endl;
+
+    return 0;
+}