Construct Tree From Given Inorder and Preorder traversals in C

March 28, 2023

Construct Tree From Given Inorder and Preorder Traversals

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.

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

Tree From Given Inorder and Preorder Traversel

Algorithm :

Pick an element from Preorder. Increment a Preorder Index Variable (preIndex in below code) to pick the next element in the next recursive call.
Create a new tree node tNode with the data as the picked element.
Find the picked element’s index in Inorder. Let the index be inIndex.
Call buildTree for elements before inIndex and make the built tree as a left subtree of tNode.
Call buildTree for elements after inIndex and make the built tree as a right subtree of tNode.
return tNode.

Tree Traversals: Inorder Postorder Preorder

BST: Binary Search Tree Program

BST: Search a node in Binary Search Tree

Construct Tree from given Inorder and Postorder traversals

Construct Tree from given Postorder and Preorder traversal

C Program based on above Algorithm

Run

#include<stdio.h>
#include<stdlib.h>
struct node
{
  char data;
  struct node *left;
  struct node *right;
};

int search (char arr[], int strt, int end, char value);
struct node *newNode (char data);

struct node *buildTree (char in[], char pre[], int inStrt, int inEnd)
{
  static int preIndex = 0;

  if (inStrt > inEnd)
    return NULL;

  struct node *tNode = newNode (pre[preIndex++]);

  if (inStrt == inEnd)
    return tNode;

  int inIndex = search (in, inStrt, inEnd, tNode->data);

  tNode->left = buildTree (in, pre, inStrt, inIndex - 1);
  tNode->right = buildTree (in, pre, inIndex + 1, inEnd);

  return tNode;
}

int search (char arr[], int strt, int end, char value)
{
  int i;
  for (i = strt; i <= end; i++)
    {
      if (arr[i] == value)
	return i;
    }
}

struct node *newNode (char data)
{
  struct node *node = (struct node *) malloc (sizeof (struct node));
  node->data = data;
  node->left = NULL;
  node->right = NULL;

  return (node);
}

void printInorder (struct node *node)
{
  if (node == NULL)
    return;

  printInorder (node->left);

  printf ("%c ", node->data);

  printInorder (node->right);
}

int main ()
{
  char in[] = { 'D', 'B', 'E', 'A', 'F', 'C' };
  char pre[] = { 'A', 'B', 'D', 'E', 'C', 'F' };
  int len = sizeof (in) / sizeof (in[0]);
  struct node *root = buildTree (in, pre, 0, len - 1);

  printf ("Inorder traversal of the constructed tree is \n");
  printInorder (root);
  getchar ();
}

Output:

Inorder traversal of the constructed tree is 
D B E A F C

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