adding the iterative approach using stacks

lectures/21_trees_IV/postorder_iterative.cpp

@@ -0,0 +1,50 @@
+#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 postorderTraversal(TreeNode* root) {
+    if (root == nullptr) return;
+
+    std::stack<TreeNode*> st1, st2;
+    st1.push(root);
+
+    while (!st1.empty()) {
+        TreeNode* current = st1.top();
+        st1.pop();
+        st2.push(current);
+
+        if (current->left) st1.push(current->left);
+        if (current->right) st1.push(current->right);
+    }
+
+    while (!st2.empty()) {
+        std::cout << st2.top()->val << " ";
+        st2.pop();
+    }
+}
+
+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: ";
+    postorderTraversal(root);
+    std::cout << std::endl;
+
+    return 0;
+}