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Iterative Preorder Traversal
• Difficulty Level : Easy
• Last Updated : 21 Dec, 2020

Given a Binary Tree, write an iterative function to print Preorder traversal of the given binary tree.
Refer this for recursive preorder traversal of Binary Tree. To convert an inherently recursive procedures to iterative, we need an explicit stack. Following is a simple stack based iterative process to print Preorder traversal.
1) Create an empty stack nodeStack and push root node to stack.
2) Do following while nodeStack is not empty.
….a) Pop an item from stack and print it.
….b) Push right child of popped item to stack
….c) Push left child of popped item to stack
Right child is pushed before left child to make sure that left subtree is processed first.

## C++

 `// C++ program to implement iterative preorder traversal` `#include `   `using` `namespace` `std;`   `/* A binary tree node has data, left child and right child */` `struct` `node {` `    ``int` `data;` `    ``struct` `node* left;` `    ``struct` `node* right;` `};`   `/* Helper function that allocates a new node with the given data and` `   ``NULL left and right  pointers.*/` `struct` `node* newNode(``int` `data)` `{` `    ``struct` `node* node = ``new` `struct` `node;` `    ``node->data = data;` `    ``node->left = NULL;` `    ``node->right = NULL;` `    ``return` `(node);` `}`   `// An iterative process to print preorder traversal of Binary tree` `void` `iterativePreorder(node* root)` `{` `    ``// Base Case` `    ``if` `(root == NULL)` `        ``return``;`   `    ``// Create an empty stack and push root to it` `    ``stack nodeStack;` `    ``nodeStack.push(root);`   `    ``/* Pop all items one by one. Do following for every popped item` `       ``a) print it` `       ``b) push its right child` `       ``c) push its left child` `    ``Note that right child is pushed first so that left is processed first */` `    ``while` `(nodeStack.empty() == ``false``) {` `        ``// Pop the top item from stack and print it` `        ``struct` `node* node = nodeStack.top();` `        ``printf``(``"%d "``, node->data);` `        ``nodeStack.pop();`   `        ``// Push right and left children of the popped node to stack` `        ``if` `(node->right)` `            ``nodeStack.push(node->right);` `        ``if` `(node->left)` `            ``nodeStack.push(node->left);` `    ``}` `}`   `// Driver program to test above functions` `int` `main()` `{` `    ``/* Constructed binary tree is` `            ``10` `          ``/   \` `        ``8      2` `      ``/  \    /` `    ``3     5  2` `  ``*/` `    ``struct` `node* root = newNode(10);` `    ``root->left = newNode(8);` `    ``root->right = newNode(2);` `    ``root->left->left = newNode(3);` `    ``root->left->right = newNode(5);` `    ``root->right->left = newNode(2);` `    ``iterativePreorder(root);` `    ``return` `0;` `}`

## Java

 `// Java program to implement iterative preorder traversal` `import` `java.util.Stack;`   `// A binary tree node` `class` `Node {`   `    ``int` `data;` `    ``Node left, right;`   `    ``Node(``int` `item)` `    ``{` `        ``data = item;` `        ``left = right = ``null``;` `    ``}` `}`   `class` `BinaryTree {`   `    ``Node root;`   `    ``void` `iterativePreorder()` `    ``{` `        ``iterativePreorder(root);` `    ``}`   `    ``// An iterative process to print preorder traversal of Binary tree` `    ``void` `iterativePreorder(Node node)` `    ``{`   `        ``// Base Case` `        ``if` `(node == ``null``) {` `            ``return``;` `        ``}`   `        ``// Create an empty stack and push root to it` `        ``Stack nodeStack = ``new` `Stack();` `        ``nodeStack.push(root);`   `        ``/* Pop all items one by one. Do following for every popped item` `         ``a) print it` `         ``b) push its right child` `         ``c) push its left child` `         ``Note that right child is pushed first so that left is processed first */` `        ``while` `(nodeStack.empty() == ``false``) {`   `            ``// Pop the top item from stack and print it` `            ``Node mynode = nodeStack.peek();` `            ``System.out.print(mynode.data + ``" "``);` `            ``nodeStack.pop();`   `            ``// Push right and left children of the popped node to stack` `            ``if` `(mynode.right != ``null``) {` `                ``nodeStack.push(mynode.right);` `            ``}` `            ``if` `(mynode.left != ``null``) {` `                ``nodeStack.push(mynode.left);` `            ``}` `        ``}` `    ``}`   `    ``// driver program to test above functions` `    ``public` `static` `void` `main(String args[])` `    ``{` `        ``BinaryTree tree = ``new` `BinaryTree();` `        ``tree.root = ``new` `Node(``10``);` `        ``tree.root.left = ``new` `Node(``8``);` `        ``tree.root.right = ``new` `Node(``2``);` `        ``tree.root.left.left = ``new` `Node(``3``);` `        ``tree.root.left.right = ``new` `Node(``5``);` `        ``tree.root.right.left = ``new` `Node(``2``);` `        ``tree.iterativePreorder();` `    ``}` `}`   `// This code has been contributed by Mayank Jaiswal`

## Python

 `# Python program to perform iterative preorder traversal`   `# A binary tree node` `class` `Node:`   `    ``# Constructor to create a new node` `    ``def` `__init__(``self``, data):` `        ``self``.data ``=` `data` `        ``self``.left ``=` `None` `        ``self``.right ``=` `None`   `# An iterative process to print preorder traveral of BT` `def` `iterativePreorder(root):` `    `  `    ``# Base CAse ` `    ``if` `root ``is` `None``:` `        ``return`   `    ``# create an empty stack and push root to it` `    ``nodeStack ``=` `[]` `    ``nodeStack.append(root)`   `    ``# Pop all items one by one. Do following for every popped item` `    ``# a) print it` `    ``# b) push its right child` `    ``# c) push its left child` `    ``# Note that right child is pushed first so that left` `    ``# is processed first */` `    ``while``(``len``(nodeStack) > ``0``):` `        `  `        ``# Pop the top item from stack and print it` `        ``node ``=` `nodeStack.pop()` `        ``print` `node.data,` `        `  `        ``# Push right and left children of the popped node` `        ``# to stack` `        ``if` `node.right ``is` `not` `None``:` `            ``nodeStack.append(node.right)` `        ``if` `node.left ``is` `not` `None``:` `            ``nodeStack.append(node.left)` `    `  `# Driver program to test above function` `root ``=` `Node(``10``)` `root.left ``=` `Node(``8``)` `root.right ``=` `Node(``2``)` `root.left.left ``=` `Node(``3``)` `root.left.right ``=` `Node(``5``)` `root.right.left ``=` `Node(``2``)` `iterativePreorder(root)`   `# This code is contributed by Nikhil Kumar Singh(nickzuck_007)`

## C#

 `// C# program to implement iterative` `// preorder traversal` `using` `System;` `using` `System.Collections.Generic;`   `// A binary tree node` `public` `class` `Node {` `    ``public` `int` `data;` `    ``public` `Node left, right;`   `    ``public` `Node(``int` `item)` `    ``{` `        ``data = item;` `        ``left = right = ``null``;` `    ``}` `}`   `class` `GFG {` `    ``public` `Node root;`   `    ``public` `virtual` `void` `iterativePreorder()` `    ``{` `        ``iterativePreorder(root);` `    ``}`   `    ``// An iterative process to print preorder` `    ``// traversal of Binary tree` `    ``public` `virtual` `void` `iterativePreorder(Node node)` `    ``{`   `        ``// Base Case` `        ``if` `(node == ``null``) {` `            ``return``;` `        ``}`   `        ``// Create an empty stack and push root to it` `        ``Stack nodeStack = ``new` `Stack();` `        ``nodeStack.Push(root);`   `        ``/* Pop all items one by one. Do following` `       ``for every popped item ` `    ``a) print it ` `    ``b) push its right child ` `    ``c) push its left child ` `    ``Note that right child is pushed first so ` `    ``that left is processed first */` `        ``while` `(nodeStack.Count > 0) {`   `            ``// Pop the top item from stack and print it` `            ``Node mynode = nodeStack.Peek();` `            ``Console.Write(mynode.data + ``" "``);` `            ``nodeStack.Pop();`   `            ``// Push right and left children of` `            ``// the popped node to stack` `            ``if` `(mynode.right != ``null``) {` `                ``nodeStack.Push(mynode.right);` `            ``}` `            ``if` `(mynode.left != ``null``) {` `                ``nodeStack.Push(mynode.left);` `            ``}` `        ``}` `    ``}`   `    ``// Driver Code` `    ``public` `static` `void` `Main(``string``[] args)` `    ``{` `        ``GFG tree = ``new` `GFG();` `        ``tree.root = ``new` `Node(10);` `        ``tree.root.left = ``new` `Node(8);` `        ``tree.root.right = ``new` `Node(2);` `        ``tree.root.left.left = ``new` `Node(3);` `        ``tree.root.left.right = ``new` `Node(5);` `        ``tree.root.right.left = ``new` `Node(2);` `        ``tree.iterativePreorder();` `    ``}` `}`   `// This code is contributed by Shrikant13`

Output:

`10 8 3 5 2 2`

Time Complexity: O(N)
Auxiliary Space: O(N), where N is the total number of nodes in the tree.

Space Optimized Solution: The idea is to start traversing the tree from root node, and keep printing the left child while exists and simultaneously, push right child of every node in an auxiliary stack. Once we reach a null node, pop a right child from the auxiliary stack and repeat the process while the auxiliary stack is not-empty.

Below is the implementation of the above approach:

## C++

 `#include ` `using` `namespace` `std;`   `// Tree Node` `struct` `Node {` `    ``int` `data;` `    ``Node *left, *right;`   `    ``Node(``int` `data)` `    ``{` `        ``this``->data = data;` `        ``this``->left = ``this``->right = NULL;` `    ``}` `};`   `// Iterative function to do Preorder traversal of the tree` `void` `preorderIterative(Node* root)` `{` `    ``if` `(root == NULL)` `        ``return``;`   `    ``stack st;`   `    ``// start from root node (set current node to root node)` `    ``Node* curr = root;`   `    ``// run till stack is not empty or current is` `    ``// not NULL` `    ``while` `(!st.empty() || curr != NULL) {` `        ``// Print left children while exist` `        ``// and keep pushing right into the` `        ``// stack.` `        ``while` `(curr != NULL) {` `            ``cout << curr->data << ``" "``;`   `            ``if` `(curr->right)` `                ``st.push(curr->right);`   `            ``curr = curr->left;` `        ``}`   `        ``// We reach when curr is NULL, so We` `        ``// take out a right child from stack` `        ``if` `(st.empty() == ``false``) {` `            ``curr = st.top();` `            ``st.pop();` `        ``}` `    ``}` `}`   `// Driver Code` `int` `main()` `{` `    ``Node* root = ``new` `Node(10);` `    ``root->left = ``new` `Node(20);` `    ``root->right = ``new` `Node(30);` `    ``root->left->left = ``new` `Node(40);` `    ``root->left->left->left = ``new` `Node(70);` `    ``root->left->right = ``new` `Node(50);` `    ``root->right->left = ``new` `Node(60);` `    ``root->left->left->right = ``new` `Node(80);`   `    ``preorderIterative(root);`   `    ``return` `0;` `}`

## Java

 `import` `java.util.Stack;`   `// A binary tree node` `class` `Node` `{` `    ``int` `data;` `    ``Node left, right;`   `    ``Node(``int` `item)` `    ``{` `        ``data = item;` `        ``left = right = ``null``;` `    ``}` `}`   `class` `BinaryTree{`   `Node root;`   `void` `preorderIterative()` `{` `    ``preorderIterative(root);` `}`   `// Iterative function to do Preorder` `// traversal of the tree ` `void` `preorderIterative(Node node)` `{` `    ``if` `(node == ``null``) ` `    ``{` `        ``return``;` `    ``}`   `    ``Stack st = ``new` `Stack();` `    `  `    ``// Start from root node (set curr ` `    ``// node to root node)` `    ``Node curr = node;` `    `  `    ``// Run till stack is not empty or ` `    ``// current is not NULL ` `    ``while` `(curr != ``null` `|| !st.isEmpty())` `    ``{` `        `  `        ``// Print left children while exist ` `        ``// and keep pushing right into the  ` `        ``// stack. ` `        ``while` `(curr != ``null``)` `        ``{` `            ``System.out.print(curr.data + ``" "``);` `            `  `            ``if` `(curr.right != ``null``)` `                ``st.push(curr.right);` `                `  `            ``curr = curr.left;` `        ``}` `        `  `        ``// We reach when curr is NULL, so We ` `        ``// take out a right child from stack ` `        ``if` `(!st.isEmpty()) ` `        ``{` `            ``curr = st.pop();` `        ``}` `    ``}` `}`   `// Driver code` `public` `static` `void` `main(String args[])` `{` `    ``BinaryTree tree = ``new` `BinaryTree();` `    `  `    ``tree.root = ``new` `Node(``10``);` `    ``tree.root.left = ``new` `Node(``20``);` `    ``tree.root.right = ``new` `Node(``30``);` `    ``tree.root.left.left = ``new` `Node(``40``);` `    ``tree.root.left.left.left = ``new` `Node(``70``);` `    ``tree.root.left.right = ``new` `Node(``50``);` `    ``tree.root.right.left = ``new` `Node(``60``);` `    ``tree.root.left.left.right = ``new` `Node(``80``);` `    `  `    ``tree.preorderIterative();` `}` `}`   `// This code is contributed by Vivek Singh Bhadauria`

## Python3

 `# Tree Node` `class` `Node:`   `    ``def` `__init__(``self``, data ``=` `0``):` `        ``self``.data ``=` `data` `        ``self``.left ``=` `None` `        ``self``.right ``=` `None` `    `  `# Iterative function to do Preorder traversal of the tree` `def` `preorderIterative(root):`   `    ``if` `(root ``=``=` `None``):` `        ``return`   `    ``st ``=` `[]`   `    ``# start from root node (set current node to root node)` `    ``curr ``=` `root`   `    ``# run till stack is not empty or current is ` `    ``# not NULL` `    ``while` `(``len``(st) ``or` `curr !``=` `None``):` `    `  `        ``# Print left children while exist` `        ``# and keep appending right into the ` `        ``# stack.` `        ``while` `(curr !``=` `None``):` `        `  `            ``print``(curr.data, end ``=` `" "``)`   `            ``if` `(curr.right !``=` `None``):` `                ``st.append(curr.right)`   `            ``curr ``=` `curr.left` `        `  `        ``# We reach when curr is NULL, so We` `        ``# take out a right child from stack` `        ``if` `(``len``(st) > ``0``):` `            ``curr ``=` `st[``-``1``]` `            ``st.pop()` `            `  `# Driver Code`   `root ``=` `Node(``10``)` `root.left ``=` `Node(``20``)` `root.right ``=` `Node(``30``)` `root.left.left ``=` `Node(``40``)` `root.left.left.left ``=` `Node(``70``)` `root.left.right ``=` `Node(``50``)` `root.right.left ``=` `Node(``60``)` `root.left.left.right ``=` `Node(``80``)`   `preorderIterative(root)`   `# This code is contributed by Arnab Kundu`

## C#

 `using` `System;` `using` `System.Collections.Generic;`   `// A binary tree node` `public` `class` `Node` `{` `    ``public` `int` `data;` `    ``public` `Node left, right;`   `    ``public` `Node(``int` `item)` `    ``{` `        ``data = item;` `        ``left = right = ``null``;` `    ``}` `}`   `public` `class` `BinaryTree{`   `Node root;`   `void` `preorderIterative()` `{` `    ``preorderIterative(root);` `}`   `// Iterative function to do Preorder` `// traversal of the tree ` `void` `preorderIterative(Node node)` `{` `    ``if` `(node == ``null``) ` `    ``{` `        ``return``;` `    ``}`   `    ``Stack st = ``new` `Stack();` `    `  `    ``// Start from root node (set curr ` `    ``// node to root node)` `    ``Node curr = node;` `    `  `    ``// Run till stack is not empty or ` `    ``// current is not NULL ` `    ``while` `(curr != ``null` `|| st.Count!=0)` `    ``{` `        `  `        ``// Print left children while exist ` `        ``// and keep pushing right into the  ` `        ``// stack. ` `        ``while` `(curr != ``null``)` `        ``{` `            ``Console.Write(curr.data + ``" "``);` `            `  `            ``if` `(curr.right != ``null``)` `                ``st.Push(curr.right);` `                `  `            ``curr = curr.left;` `        ``}` `        `  `        ``// We reach when curr is NULL, so We ` `        ``// take out a right child from stack ` `        ``if` `(st.Count != 0) ` `        ``{` `            ``curr = st.Pop();` `        ``}` `    ``}` `}`   `// Driver code` `public` `static` `void` `Main(String []args)` `{` `    ``BinaryTree tree = ``new` `BinaryTree();` `    `  `    ``tree.root = ``new` `Node(10);` `    ``tree.root.left = ``new` `Node(20);` `    ``tree.root.right = ``new` `Node(30);` `    ``tree.root.left.left = ``new` `Node(40);` `    ``tree.root.left.left.left = ``new` `Node(70);` `    ``tree.root.left.right = ``new` `Node(50);` `    ``tree.root.right.left = ``new` `Node(60);` `    ``tree.root.left.left.right = ``new` `Node(80);` `    `  `    ``tree.preorderIterative();` `}` `}`   `// This code is contributed by Amit Katiyar`

Output:

`10 20 40 70 80 50 30 60`

Time Complexity: O(N)
Auxiliary Space: O(H), where H is the height of the tree.