Inorder Tree Traversal without Recursion

Using Stack is the obvious way to traverse tree without recursion. Below is an algorithm for traversing binary tree using stack. See this for step wise step execution of the algorithm.

1) Create an empty stack S.
2) Initialize current node as root
3) Push the current node to S and set current = current->left until current is NULL
4) If current is NULL and stack is not empty then 
     a) Pop the top item from stack.
     b) Print the popped item, set current = popped_item->right 
     c) Go to step 3.
5) If current is NULL and stack is empty then we are done.

Let us consider the below tree for example

            1
          /   \
        2      3
      /  \
    4     5

Step 1 Creates an empty stack: S = NULL

Step 2 sets current as address of root: current -> 1

Step 3 Pushes the current node and set current = current->left until current is NULL
     current -> 1
     push 1: Stack S -> 1
     current -> 2
     push 2: Stack S -> 2, 1
     current -> 4
     push 4: Stack S -> 4, 2, 1
     current = NULL

Step 4 pops from S
     a) Pop 4: Stack S -> 2, 1
     b) print "4"
     c) current = NULL /*right of 4 */ and go to step 3
Since current is NULL step 3 doesn't do anything. 

Step 4 pops again.
     a) Pop 2: Stack S -> 1
     b) print "2"
     c) current -> 5/*right of 2 */ and go to step 3

Step 3 pushes 5 to stack and makes current NULL
     Stack S -> 5, 1
     current = NULL

Step 4 pops from S
     a) Pop 5: Stack S -> 1
     b) print "5"
     c) current = NULL /*right of 5 */ and go to step 3
Since current is NULL step 3 doesn't do anything

Step 4 pops again.
     a) Pop 1: Stack S -> NULL
     b) print "1"
     c) current -> 3 /*right of 1 */  

Step 3 pushes 3 to stack and makes current NULL
     Stack S -> 3
     current = NULL

Step 4 pops from S
     a) Pop 3: Stack S -> NULL
     b) print "3"
     c) current = NULL /*right of 3 */  

Traversal is done now as stack S is empty and current is NULL. 

C++

// C++ program to print inorder traversal
// using stack.
#include<bits/stdc++.h>
using namespace std;

/* A binary tree Node has data, pointer to left child
   and a pointer to right child */
struct Node
{
    int data;
    struct Node* left;
    struct Node* right;
    Node (int data)
    {
        this->data = data;
        left = right = NULL;
    }
};

/* Iterative function for inorder tree
   traversal */
void inOrder(struct Node *root)
{
    stack<Node *> s;
    Node *curr = root;

    while (curr != NULL || s.empty() == false)
    {
        /* Reach the left most Node of the
           curr Node */
        while (curr !=  NULL)
        {
            /* place pointer to a tree node on
               the stack before traversing
              the node's left subtree */
            s.push(curr);
            curr = curr->left;
        }

        /* Current must be NULL at this point */
        curr = s.top();
        s.pop();

        cout << curr->data << " ";

        /* we have visited the node and its
           left subtree.  Now, it's right
           subtree's turn */
        curr = curr->right;

    } /* end of while */
}

/* Driver program to test above functions*/
int main()
{

    /* Constructed binary tree is
              1
            /   \
          2      3
        /  \
      4     5
    */
    struct Node *root = new Node(1);
    root->left        = new Node(2);
    root->right       = new Node(3);
    root->left->left  = new Node(4);
    root->left->right = new Node(5);

    inOrder(root);
    return 0;
}

C

#include<stdio.h>
#include<stdlib.h>
#define bool int

/* A binary tree tNode has data, pointer to left child
   and a pointer to right child */
struct tNode
{
   int data;
   struct tNode* left;
   struct tNode* right;
};

/* Structure of a stack node. Linked List implementation is used for
   stack. A stack node contains a pointer to tree node and a pointer to
   next stack node */
struct sNode
{
  struct tNode *t;
  struct sNode *next;
};

/* Stack related functions */
void push(struct sNode** top_ref, struct tNode *t);
struct tNode *pop(struct sNode** top_ref);
bool isEmpty(struct sNode *top);

/* Iterative function for inorder tree traversal */
void inOrder(struct tNode *root)
{
  /* set current to root of binary tree */
  struct tNode *current = root;
  struct sNode *s = NULL;  /* Initialize stack s */
  bool done = 0;

  while (!done)
  {
    /* Reach the left most tNode of the current tNode */
    if(current !=  NULL)
    {
      /* place pointer to a tree node on the stack before traversing
        the node's left subtree */
      push(&s, current);
      current = current->left;
    }

    /* backtrack from the empty subtree and visit the tNode
       at the top of the stack; however, if the stack is empty,
      you are done */
    else
    {
      if (!isEmpty(s))
      {
        current = pop(&s);
        printf("%d ", current->data);

        /* we have visited the node and its left subtree.
          Now, it's right subtree's turn */
        current = current->right;
      }
      else
        done = 1;
    }
  } /* end of while */
}

/* UTILITY FUNCTIONS */
/* Function to push an item to sNode*/
void push(struct sNode** top_ref, struct tNode *t)
{
  /* allocate tNode */
  struct sNode* new_tNode =
            (struct sNode*) malloc(sizeof(struct sNode));

  if(new_tNode == NULL)
  {
     printf("Stack Overflow \n");
     getchar();
     exit(0);
  }

  /* put in the data  */
  new_tNode->t  = t;

  /* link the old list off the new tNode */
  new_tNode->next = (*top_ref);

  /* move the head to point to the new tNode */
  (*top_ref)    = new_tNode;
}

/* The function returns true if stack is empty, otherwise false */
bool isEmpty(struct sNode *top)
{
   return (top == NULL)? 1 : 0;
}

/* Function to pop an item from stack*/
struct tNode *pop(struct sNode** top_ref)
{
  struct tNode *res;
  struct sNode *top;

  /*If sNode is empty then error */
  if(isEmpty(*top_ref))
  {
     printf("Stack Underflow \n");
     getchar();
     exit(0);
  }
  else
  {
     top = *top_ref;
     res = top->t;
     *top_ref = top->next;
     free(top);
     return res;
  }
}

/* Helper function that allocates a new tNode with the
   given data and NULL left and right pointers. */
struct tNode* newtNode(int data)
{
  struct tNode* tNode = (struct tNode*)
                       malloc(sizeof(struct tNode));
  tNode->data = data;
  tNode->left = NULL;
  tNode->right = NULL;

  return(tNode);
}

/* Driver program to test above functions*/
int main()
{

  /* Constructed binary tree is
            1
          /   \
        2      3
      /  \
    4     5
  */
  struct tNode *root = newtNode(1);
  root->left        = newtNode(2);
  root->right       = newtNode(3);
  root->left->left  = newtNode(4);
  root->left->right = newtNode(5);

  inOrder(root);

  getchar();
  return 0;
}

Java

// non-recursive java program for inorder traversal
import java.util.Stack;

/* Class containing left and right child of
current node and key value*/
class Node
{
    int data;
    Node left, right;

    public Node(int item)
    {
        data = item;
        left = right = null;
    }
}

/* Class to print the inorder traversal */
class BinaryTree
{
    Node root;
    void inorder()
    {
        if (root == null)
            return;


        Stack<Node> s = new Stack<Node>();
        Node curr = root;

        // traverse the tree
        while (curr != null || s.size() > 0)
        {

            /* Reach the left most Node of the
            curr Node */
            while (curr !=  null)
            {
                /* place pointer to a tree node on
                   the stack before traversing
                  the node's left subtree */
                s.push(curr);
                curr = curr.left;
            }

            /* Current must be NULL at this point */
            curr = s.pop();

            System.out.print(curr.data + " ");

            /* we have visited the node and its
               left subtree.  Now, it's right
               subtree's turn */
            curr = curr.right;
        }
    }

    public static void main(String args[])
    {

        /* creating a binary tree and entering
        the nodes */
        BinaryTree tree = new BinaryTree();
        tree.root = new Node(1);
        tree.root.left = new Node(2);
        tree.root.right = new Node(3);
        tree.root.left.left = new Node(4);
        tree.root.left.right = new Node(5);
        tree.inorder();
    }
}

Python

# Python program to do inorder traversal without recursion

# 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

# Iterative function for inorder tree traversal
def inOrder(root):

    # Set current to root of binary tree
    current = root
    stack = [] # initialize stack
    done = 0

    while True:

        # Reach the left most Node of the current Node
        if current is not None:

            # Place pointer to a tree node on the stack
            # before traversing the node's left subtree
            stack.append(current)

            current = current.left


        # BackTrack from the empty subtree and visit the Node
        # at the top of the stack; however, if the stack is
        # empty you are done
        elif(stack):
            current = stack.pop()
            print(current.data, end=" ") # Python 3 printing

            # We have visited the node and its left
            # subtree. Now, it's right subtree's turn
            current = current.right

        else:
            break

    print()

# Driver program to test above function

""" Constructed binary tree is
            1
          /   \
         2     3
       /  \
      4    5   """

root = Node(1)
root.left = Node(2)
root.right = Node(3)
root.left.left = Node(4)
root.left.right = Node(5)

inOrder(root)

# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

C#

// Non-recursive C# program for inorder traversal
using System;
using System.Collections.Generic;


/* Class containing left and right child of
current node and key value*/
public class Node
{
    public int data;
    public Node left, right;

    public Node(int item)
    {
        data = item;
        left = right = null;
    }
}

/* Class to print the inorder traversal */
public class BinaryTree
{
    public Node root;
    public virtual void inorder()
    {
        if (root == null)
        {
            return;
        }


        Stack<Node> s = new Stack<Node>();
        Node curr = root;

        // traverse the tree
        while (curr != null || s.Count > 0)
        {

            /* Reach the left most Node of the
            curr Node */
            while (curr != null)
            {
                /* place pointer to a tree node on
                   the stack before traversing
                  the node's left subtree */
                s.Push(curr);
                curr = curr.left;
            }

            /* Current must be NULL at this point */
            curr = s.Pop();

            Console.Write(curr.data + " ");

            /* we have visited the node and its
               left subtree.  Now, it's right
               subtree's turn */
            curr = curr.right;
        }
    }

    public static void Main(string[] args)
    {

        /* creating a binary tree and entering
        the nodes */
        BinaryTree tree = new BinaryTree();
        tree.root = new Node(1);
        tree.root.left = new Node(2);
        tree.root.right = new Node(3);
        tree.root.left.left = new Node(4);
        tree.root.left.right = new Node(5);
        tree.inorder();
    }
}

// This code is contributed by Shrikant13

Output:

 4 2 5 1 3

Time Complexity: O(n)

References:
http://web.cs.wpi.edu/~cs2005/common/iterative.inorder
http://neural.cs.nthu.edu.tw/jang/courses/cs2351/slide/animation/Iterative%20Inorder%20Traversal.pps

See this post for another approach of Inorder Tree Traversal without recursion and without stack!

Please write comments if you find any bug in above code/algorithm, or want to share more information about stack based Inorder Tree Traversal.

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Improved By : Rishabh Jindal 2, shrikanth13, christineehill, iamaerojex