Construct Tree from given Postorder and Inorder Traversals

Construct Tree From Given Inorder and Postorder Traversals in Java

There are three types of traversals in a tree: Inorder, Preorder and Postorder traversal. A tree can be formed with any two tree traversals in which one of them being the in order traversal.

construct a tree from preorder and inorder traversal

Construct Tree From Given Inorder and Postorder Traversals

Example

Given traversals:
Postorder: 12, 30, 40, 37, 25, 60, 70, 62, 87, 75, 50
Inorder:12, 25, 30, 37, 40, 50, 60, 62, 70, 75, 87
1. The last element in the postorder traversal is the root of the tree.
So, here 50 will be the root of the tree.
2. We will find the index of 50 in the inorder traversal.The index found is 5. Let this index is denoted by ‘pos’.
3. All the elements to the left of this index ( from 0 to 4 index) will be in the left subtree of 50.
4. And all the elements to the right of this index ( from 6 to 10) will be in the right subtree of 50.

structure of tree

Now, we will divide preorder and inorder array in two parts.
One is for the left subtree and other is for the right subtree.

Let

  • psi: starting index for preorder array
  • pei: ending index for preorder array
  • isi: starting index of inorder array
  • iei: ending index of preorder array
  • clc: Number of elements in the left subtree

Clearly, clc= pos – isi;

For left subtree:
Postorder array: from index psi, psi + clc – 1
Inorder array: from index isi, isi + clc -1

For right subtree:
Postorder array: from index psi+clc, pei – 1
Inorder array: from index isi + clc + 1, iei


Using the above arrays, all the steps are recursively repeated.
The following binary tree is obtained:

Construct Tree from given Inorder and Postorder traversals

Code For Binary Search Tree​

Run
public class Main
{

// Binary tree class
  public static class BinaryTree
  {
// Node class
    public class Node
    {
      int data;
      Node left;
      Node right;

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

    }

    private Node root;
    private int size;

    public BinaryTree (int[]post, int[]in)
    {
      this.root =
	this.construct (post, 0, post.length -1, in, 0, in.length-1);

    }

    private Node construct (int[]post, int psi, int pei, int[]in, int isi,
			    int iei)
    {

// Base case
      if (psi > pei)
	{
	  return null;
	}

      Node node = new Node (post[pei]);
      node.left = null;
      node.right = null;
      this.size++;

// Searching post[pei] in inorder array
      int pos = -1;
      for (int i = isi; i <= iei; i++)
	{
	  if (in[i] == node.data)
	    {
	      pos = i;
	      break;
	    }
	}

// Number of elements in left subtree
      int clc = pos-isi;

// Left subtree
      node.left =
	this.construct (post, psi, psi + clc-1, in, isi, isi + clc-1);

// Right subtree
      node.right =
	this.construct (post, psi + clc, pei-1, in, isi + clc + 1, iei);

      return node;
    }

// Postorder tree traversal
    public void postOrder ()
    {
      postOrder (this.root);
    }

    private void postOrder (Node node)
    {
      if (node == null)
	{
	  return;
	}

      postOrder (node.left);
      postOrder (node.right);
      System.out.print (node.data + " ");
    }
  }

  public static void main (String[]args) throws Exception
  {

// Construct binary tree

    int[] in = { 12, 25, 30, 37, 40, 50, 60, 62, 70, 75, 87 };
    int[] post = { 12, 30, 40, 37, 25, 60, 70, 62, 87, 75, 50 };

    BinaryTree bt = new BinaryTree (post, in);
    System.out.println("The new tree constructed is (Postorder) :  ");
    bt.postOrder ();

  }
}

Output :

The new tree constructed is (Postorder) :  
12 30 40 37 25 60 70 62 87 75 50 

Prime Course Trailer

Related Banners

Get PrepInsta Prime & get Access to all 200+ courses offered by PrepInsta in One Subscription

Get over 200+ course One Subscription

Courses like AI/ML, Cloud Computing, Ethical Hacking, C, C++, Java, Python, DSA (All Languages), Competitive Coding (All Languages), TCS, Infosys, Wipro, Amazon, DBMS, SQL and others

Checkout list of all the video courses in PrepInsta Prime Subscription

Checkout list of all the video courses in PrepInsta Prime Subscription

Introduction to Trees

Binary Trees

Binary Search Trees

Traversals

  • Traversal in Trees
  • Tree Traversals: Breadth-First Search (BFS) : C | C++ | Java
  • Tree Traversals: Depth First Search (DFS) : C | C++ | Java
  • Construct a Binary Tree from Postorder and Inorder

B – Trees

AVL Trees

  • AVL Trees
    • AVL Trees: Introduction
    • AVL Tree Insertion : C | C++ | Java
    • AVL Tree Deletion : C | C++ | Java
    • Insertion in a Binary Tree (Level Order) – C | C++ | Java
    • Searching in Binary Tree – C | C++ | Java
    • Searching in a Binary Search Tree – C | C++ | Java

Complete Programs for Trees

  • Depth First Traversals – C | C++ | Java
  • Level Order Traversal – C | C++Java
  • Construct Tree from given Inorder and Preorder traversals – C | C++Java
  • Construct Tree from given Postorder and Inorder traversals – C | C++Java
  • Construct Tree from given Postorder and Preorder traversal – C | C++Java
  • Find size of the Binary tree – C | C++Java
  • Find the height of binary tree – C | C++Java
  • Find maximum in binary tree – C | C++Java
  • Check whether two tree are identical- CC++Java
  • Spiral Order traversal of Tree- CC++Java
  • Level Order Traversal Line by Line – C | C++Java
  • Hand shaking lemma and some Impotant Tree Properties.
  • Check If binary tree if Foldable or not.- CC++Java
  • check whether tree is Symmetric – C| C++Java.
  • Check for Children-Sum in Binary Tree- C|C++Java
  • Sum of all nodes in Binary Tree- CC++ | Java
  • Lowest Common Ancestor in Binary Tree- CC++ | Java

Introduction to Trees

Binary Trees

Binary Search Trees

Traversals

  • Traversal in Trees
  • Tree Traversals: Breadth-First Search (BFS) : C | C++ | Java
  • Tree Traversals: Depth First Search (DFS) : C | C++ | Java
  • Construct a Binary Tree from Postorder and Inorder

B – Trees

AVL Trees

  • AVL Trees
    • AVL Trees: Introduction
    • AVL Tree Insertion :  C | C++ | Java
    • AVL Tree Deletion : C | C++ | Java
    • Insertion in a Binary Tree (Level Order) – C | C++ | Java
    • Searching in Binary Tree – C | C++ | Java
    • Searching in a Binary Search Tree – C | C++ | Java

Complete Programs for Trees

  • Depth First Traversals – C | C++ | Java
  • Level Order Traversal – C | C++Java
  • Construct Tree from given Inorder and Preorder traversals – C | C++Java
  • Construct Tree from given Postorder and Inorder traversals – C | C++Java
  • Construct Tree from given Postorder and Preorder traversal – C | C++Java
  • Find size of the Binary tree – C | C++Java
  • Find the height of binary tree – C | C++Java
  • Find maximum in binary tree – C | C++Java
  • Check whether two tree are identical- CC++Java
  • Spiral Order traversal of Tree- CC++Java
  • Level Order Traversal LIne by Line – C | C++Java
  • Hand shaking lemma and some Impotant Tree Properties.
  • Check If binary tree if Foldable or not.- CC++Java
  • check whether tree is Symmetric  C| C++Java.
  • Check for Children-Sum in Binary Tree- C|C++Java
  • Sum of all nodes in Binary Tree- CC++ | Java
  • Lowest Common Ancestor in Binary Tree. CC++ | Java

Construct Tree from given Postorder and Inorder traversals in C

Construct Tree from given Postorder and Inorder Traversals

On this page we wiil discuss about how to construct tree from given postorder and inorder traversals in C .

There are three types of traversals in a tree: Inorder, Preorder and Postorder traversal. A tree can be formed with any two tree traversals in which one of them being the in order traversal.

Postorder Traversal: We first  move to the left subtree and then to the right subtree and finally print the node.
Inorder Traversal: We move to the left subtree, then print the node and move to the right subtree.

postorder inorder traversal

Tree From Given Postorder and Inorder Traversal

Algorithm For InOrder Traversal

  • Traverse The Left subtree.
  • Print the node.
  • Traverse the right subtree.

Algorithm For PostOrder Traversal

  • Traverse the left subtree.
  • Traverse the right subtree.
  • Print the node.

Algorithm for tree construction:

  • Start with root node, which will be the last element in the postorder sequence
  •  And find the boundary of its left and right subtree in the inorder sequence.
  • To find the boundary, search for the index of the root node in the inorder sequence.
  • All keys before the root node in the inorder sequence become part of the left subtree, and all the keys after the root node become part of the right subtree.
  • Repeat the above step recursively for all the nodes in the tree and construct the tree.
Construct Tree from given Postorder and Inorder Traversals in C++

Code in C to Construct Tree from given Postorder and Inorder Traversals

Run
#include <stdio.h>
#include <stdlib.h>
 
// Data structure to store a binary tree node
struct Node
{
    int key;
    struct Node *left, *right;
};
 
// Function to create a new binary tree node having a given key
struct Node* newNode(int key)
{
    struct Node* node = (struct Node*)malloc(sizeof(struct Node));
    node->key = key;
    node->left = node->right = NULL;
 
    return node;
}
 
// Recursive function to perform inorder traversal on a given binary tree
void inorderTraversal(struct Node* root)
{
    if (root == NULL) {
        return;
    }
 
    inorderTraversal(root->left);
    printf("%d ", root->key);
    inorderTraversal(root->right);
}
 
// Recursive function to perform postorder traversal on a given binary tree
void postorderTraversal(struct Node* root)
{
    if (root == NULL) {
        return;
    }
 
    postorderTraversal(root->left);
    postorderTraversal(root->right);
    printf("%d ", root->key);
}
 
// Recursive function to construct a binary tree from a given
// inorder and postorder traversals
struct Node* construct(int inorder[], int start, int end,
                    int postorder[], int *pIndex)
{
    // base case
    if (start > end) {
        return NULL;
    }
 
    // Consider the next item from the end of a given postorder sequence.
    // This value would be the root node of the subtree formed by the sequence
    // inorder[start, end].
    struct Node* node = newNode(postorder[(*pIndex)--]);
 
    // search the current node index in inorder sequence to determine
    // the boundary of the left and right subtree of the current node
    int i;
    for (i = start; i <= end; i++)
    {
        if (inorder[i] == node->key) {
            break;
        }
    }
 
    // recursively construct the right subtree
    node->right= construct(inorder, i + 1, end, postorder, pIndex);
 
    // recursively construct the left subtree
    node->left = construct(inorder, start, i - 1, postorder, pIndex);
 
    // return the current node
    return node;
}
 
// Construct a binary tree from inorder and postorder traversals.
// This function assumes that the input is valid, i.e., given
// inorder and postorder sequences forming a binary tree.
struct Node* constructTree(int inorder[], int postorder[], int n)
{
    // `pIndex` stores the index of the next unprocessed node from the end
    // of the postorder sequence
    int *pIndex = &n;
    return construct(inorder, 0, n, postorder, pIndex);
}
 
int main(void)
{
    /* Construct the following tree
               1
             /   \
            /     \
           2       3
          /       / \
         /       /   \
        4       5     6
               / \
              /   \
             7     8
    */
 
    int inorder[]    = { 4, 2, 1, 7, 5, 8, 3, 6 };
    int postorder[] = { 4, 2, 7, 8, 5, 6, 3, 1 };
    int n = sizeof(inorder)/sizeof(inorder[0]);
 
    struct Node* root = constructTree(inorder, postorder, n - 1);
 
    // traverse the constructed tree
    printf("Inorder traversal is "); inorderTraversal(root);
    printf("\nPostorder traversal is "); postorderTraversal(root);
 
    return 0;
}

Output:

Inorder traversal is 4 2 1 7 5 8 3 6 
Postorder traversal is 4 2 7 8 5 6 3 1 

Prime Course Trailer

Related Banners

Get PrepInsta Prime & get Access to all 200+ courses offered by PrepInsta in One Subscription

Get over 200+ course One Subscription

Courses like AI/ML, Cloud Computing, Ethical Hacking, C, C++, Java, Python, DSA (All Languages), Competitive Coding (All Languages), TCS, Infosys, Wipro, Amazon, DBMS, SQL and others

Checkout list of all the video courses in PrepInsta Prime Subscription

Checkout list of all the video courses in PrepInsta Prime Subscription

Introduction to Trees

Binary Trees

Binary Search Trees

Traversals

  • Traversal in Trees
  • Tree Traversals: Breadth-First Search (BFS) : C | C++ | Java
  • Tree Traversals: Depth First Search (DFS) : C | C++ | Java
  • Construct a Binary Tree from Postorder and Inorder

B – Trees

AVL Trees

  • AVL Trees
    • AVL Trees: Introduction
    • AVL Tree Insertion : C | C++ | Java
    • AVL Tree Deletion : C | C++ | Java
    • Insertion in a Binary Tree (Level Order) – C | C++ | Java
    • Searching in Binary Tree – C | C++ | Java
    • Searching in a Binary Search Tree – C | C++ | Java

Complete Programs for Trees

  • Depth First Traversals – C | C++ | Java
  • Level Order Traversal – C | C++Java
  • Construct Tree from given Inorder and Preorder traversals – C | C++Java
  • Construct Tree from given Postorder and Inorder traversals – C | C++Java
  • Construct Tree from given Postorder and Preorder traversal – C | C++Java
  • Find size of the Binary tree – C | C++Java
  • Find the height of binary tree – C | C++Java
  • Find maximum in binary tree – C | C++Java
  • Check whether two tree are identical- CC++Java
  • Spiral Order traversal of Tree- CC++Java
  • Level Order Traversal Line by Line – C | C++Java
  • Hand shaking lemma and some Impotant Tree Properties.
  • Check If binary tree if Foldable or not.- CC++Java
  • check whether tree is Symmetric – C| C++Java.
  • Check for Children-Sum in Binary Tree- C|C++Java
  • Sum of all nodes in Binary Tree- CC++ | Java
  • Lowest Common Ancestor in Binary Tree- CC++ | Java

Introduction to Trees

Binary Trees

Binary Search Trees

Traversals

  • Traversal in Trees
  • Tree Traversals: Breadth-First Search (BFS) : C | C++ | Java
  • Tree Traversals: Depth First Search (DFS) : C | C++ | Java
  • Construct a Binary Tree from Postorder and Inorder

B – Trees

AVL Trees

  • AVL Trees
    • AVL Trees: Introduction
    • AVL Tree Insertion :  C | C++ | Java
    • AVL Tree Deletion : C | C++ | Java
    • Insertion in a Binary Tree (Level Order) – C | C++ | Java
    • Searching in Binary Tree – C | C++ | Java
    • Searching in a Binary Search Tree – C | C++ | Java

Complete Programs for Trees

  • Depth First Traversals – C | C++ | Java
  • Level Order Traversal – C | C++Java
  • Construct Tree from given Inorder and Preorder traversals – C | C++Java
  • Construct Tree from given Postorder and Inorder traversals – C | C++Java
  • Construct Tree from given Postorder and Preorder traversal – C | C++Java
  • Find size of the Binary tree – C | C++Java
  • Find the height of binary tree – C | C++Java
  • Find maximum in binary tree – C | C++Java
  • Check whether two tree are identical- CC++Java
  • Spiral Order traversal of Tree- CC++Java
  • Level Order Traversal LIne by Line – C | C++Java
  • Hand shaking lemma and some Impotant Tree Properties.
  • Check If binary tree if Foldable or not.- CC++Java
  • check whether tree is Symmetric  C| C++Java.
  • Check for Children-Sum in Binary Tree- C|C++Java
  • Sum of all nodes in Binary Tree- CC++ | Java
  • Lowest Common Ancestor in Binary Tree. CC++ | Java