update the non-recursion ways of binary tree

Binary Tree Algorithms/Non-Recursion Traversal Algorithms/README.md

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+# Non-Recursive Binary Tree Traversal
+
+This section describes the implementation of functions in C that performs non-recursive traversal of a binary tree. Here offers pre-order, in-order, and post-order traversals.
+
+## Problem Statement
+Given a binary tree, implement pre-order, in-order, and post-order traversals of the tree in a non-recursive manner.
+
+## Solution
+To perform non-recursive traversal of a binary tree, we utilize a stack data structure. The `StackNode` struct is essential as it allows us to keep track of the nodes during traversal. Each `StackNode` contains a pointer to a `Node` of the binary tree and a pointer to the next `StackNode`, mimicking the Last-In-First-Out (LIFO) behavior of a stack.
+
+### 1. pre-order Traversal
+In pre-order traversal, we visit the root node first, then recursively perform pre-order traversal on the left subtree, and finally on the right subtree.
+
+**Implementation Details:**
+- Initialize an empty stack and push the root node onto the stack.
+- Pop a node from the stack, visit it, and push its right child followed by its left child onto the stack.
+- Repeat the process until the stack is empty.
+
+### 2. in-order Traversal
+In in-order traversal, we recursively perform in-order traversal on the left subtree, visit the root node, and then recursively perform in-order traversal on the right subtree.
+
+**Implementation Details:**
+- Initialize an empty stack and set the current node to the root.
+- Push all left children of the current node onto the stack until a leaf node is reached.
+- Pop a node from the stack, visit it, set the current node to its right child, and repeat the process.
+- If the current node is NULL and the stack is empty, the traversal is complete.
+
+### 3. post-order Traversal
+In post-order traversal, we recursively perform post-order traversal on the left subtree, then on the right subtree, and finally visit the root node.
+
+**Implementation Details:**
+- Initialize two stacks, `stack1` and `stack2`.
+- Push the root node onto `stack1`.
+- Pop a node from `stack1`, push it onto `stack2`, and then push its left child followed by its right child onto `stack1`.
+- Repeat the process until `stack1` is empty.
+- Pop nodes from `stack2` and visit them, which will be in post-order since the last nodes to be popped from `stack1` are the leftmost nodes.

Binary Tree Algorithms/Non-Recursion Traversal Algorithms/program.c

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+#include <stdio.h>
+#include <stdlib.h>
+
+
+typedef struct Node {
+    int data;               
+    struct Node *left;      
+    struct Node *right;     
+} Node;
+
+
+typedef struct StackNode {
+    Node *treeNode;
+    // Pointer to the next stack node
+    struct StackNode *next; 
+} StackNode;
+
+// Function to create a new node
+Node* newNode(int data) {
+    Node *node = (Node *)malloc(sizeof(Node)); 
+    node->data = data;                          
+    node->left = NULL;                          
+    node->right = NULL;                         
+    return node;
+}
+
+// Function to create a new stack node
+StackNode* newStackNode(Node *treeNode) {
+    StackNode *stackNode = (StackNode *)malloc(sizeof(StackNode)); 
+    stackNode->treeNode = treeNode;  
+    stackNode->next = NULL;   
+    return stackNode;
+}
+
+// Function to check if the stack is empty
+int isStackEmpty(StackNode *top) {
+    return top == NULL;
+}
+
+// Function to push a node onto the stack
+void push(StackNode **top, Node *treeNode) {
+    StackNode *newStackNode = (StackNode *)malloc(sizeof(StackNode));
+    newStackNode->treeNode = treeNode; 
+    newStackNode->next = *top;        
+    *top = newStackNode;           
+}
+
+// Function to pop a node from the stack
+Node* pop(StackNode **top) {
+    if (isStackEmpty(*top)) {
+        return NULL;
+    }
+    StackNode *temp = *top; 
+    Node *treeNode = temp->treeNode; 
+    *top = temp->next; 
+    free(temp);        
+    return treeNode; 
+}
+
+// Function to get the top node of the stack
+Node* top(StackNode *top) {
+    if (isStackEmpty(top)) {
+        return NULL; 
+    }
+    return top->treeNode;
+}
+
+// Function to perform preorder traversal of the tree
+void preorderTraversal(Node *root) {
+    if (root == NULL) return; 
+    StackNode *stack = NULL;  
+    // Push the root node onto the stack
+    push(&stack, root);       
+    while (!isStackEmpty(stack)) {
+        Node *current = pop(&stack); 
+        // Print the data
+        printf("%d ", current->data);
+        if (current->right) push(&stack, current->right);
+        if (current->left) push(&stack, current->left); 
+    }
+}
+
+// Function to perform inorder traversal of the tree
+void inorderTraversal(Node *root) {
+    if (root == NULL) return; 
+    StackNode *stack = NULL; 
+    Node *current = root;     
+    while (current != NULL || !isStackEmpty(stack)) {
+        while (current != NULL) {
+            // Push nodes onto the stack
+            push(&stack, current); 
+            // Move to the left child
+            current = current->left; 
+        }
+        current = pop(&stack);
+        printf("%d ", current->data); 
+        // Move to the right child
+        current = current->right; 
+    }
+}
+
+// Function to perform postorder traversal of the tree
+void postorderTraversal(Node *root) {
+    if (root == NULL) return; 
+    // Initialize stack1
+    StackNode *stack1 = NULL; 
+    // Initialize stack2
+    StackNode *stack2 = NULL; 
+    // Push the root node onto stack1
+    push(&stack1, root);     
+    while (!isStackEmpty(stack1)) {
+        Node *current = pop(&stack1); 
+        // Push the node onto stack2
+        push(&stack2, current);       
+        // Push left child if it exists
+        if (current->left) push(&stack1, current->left); 
+        // Push right child if it exists
+        if (current->right) push(&stack1, current->right); 
+    }
+    while (!isStackEmpty(stack2)) {
+        // Pop a node from stack2
+        Node *current = pop(&stack2); 
+        printf("%d ", current->data);
+    }
+}
+
+// Main function to demonstrate tree traversals
+int main() {
+    Node* root = newNode(10); 
+    root->left = newNode(5); 
+    root->right = newNode(20); 
+    root->left->left = newNode(3); 
+    root->left->right = newNode(8); 
+    printf("Preorder traversal: ");
+    preorderTraversal(root); 
+    printf("\n");
+    printf("Inorder traversal: ");
+    inorderTraversal(root); 
+    printf("\n");
+    printf("Postorder traversal: ");
+    postorderTraversal(root); 
+    printf("\n");
+    return 0;
+}