adding the iterative approach using stacks

lectures/21_trees_IV/inorder_iterative.cpp

@@ -0,0 +1,47 @@
+#include <iostream>
+#include <stack>
+
+class TreeNode {
+public:
+    int val;
+    TreeNode* left;
+    TreeNode* right;
+    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+};
+
+// in order traverse a binary tree, iteratively.
+void inorderTraversal(TreeNode* root) {
+    std::stack<TreeNode*> st;
+    TreeNode* current = root;
+
+    while (current != nullptr || !st.empty()) {
+        while (current != nullptr) {  // reach leftmost node
+            st.push(current);
+            current = current->left;
+        }
+
+        current = st.top();  // process node
+        st.pop();
+        std::cout << current->val << " ";
+
+        current = current->right;  // move to right subtree
+    }
+}
+
+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: ";
+    inorderTraversal(root);
+    std::cout << std::endl;
+
+    return 0;
+}