# Recursive Program to Print extreme nodes of each level of Binary Tree in alternate order

Given a binary tree, the task is to print nodes of extreme corners of each level but in alternate order.

Examples:

Input :
1
/  \
2    3
/    /  \
4    5    6
/    / \
7    8   9
Output : 1 2 6 7
Print the rightmost node at 1st level: 1
Print the leftmost node at 2nd level: 2
Print the rightmost node at 3rd level: 6
Print the leftmost node at 4th level: 7
Other possible output will be -> 1 3 4 9

Input :
3
/  \
8    1
/ \  / \
9  5 6   4
Output : 3 8 4

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

We have already discussed the iterative approach to solve this problem. In this post the recursive approach is discussed.

Approach: The idea is to perform level order traversal in the spiral form and at each level print the first node during the traversal, these will be the nodes at extreme corner present in the alternate form.

Below is the implementation of the above approach:

## C++

 // C++ program to print nodes of extreme corners // of each level in alternate order    #include using namespace std;    // A binary tree node struct Node {     int data;     Node *left, *right; };    // Utility function to allocate memory for a new node Node* newNode(int data) {     Node* node = new (Node);     node->data = data;     node->left = node->right = NULL;     return (node); }    // Function that returns the height of the binary tree int height(Node* root) {     if (root == NULL)         return 0;        int lheight = height(root->left);     int rheight = height(root->right);        return max(lheight, rheight) + 1; }    // Function performs level order traversal from right to // left and prints the first node during the traversal void rightToLeft(Node* root, int level, int& f) {     if (root == NULL)         return;        // Checks for the value of f so that     // only first node is printed during     // the traversal and no other node is printed     if (level == 1 && f == 0) {         printf("%d ", root->data);         f = 1;     }        else if (level > 1) {         rightToLeft(root->right, level - 1, f);         rightToLeft(root->left, level - 1, f);     } }    // Function performs level order traversal from left to // right and prints the first node during the traversal void leftToRight(Node* root, int level, int& f) {     if (root == NULL)         return;        // Checks for the value of f so that     // only first node is printed during     // the traversal and no other node is printed     if (level == 1 && f == 1) {         printf("%d ", root->data);         f = 0;     }        else if (level > 1) {         leftToRight(root->left, level - 1, f);         leftToRight(root->right, level - 1, f);     } }    // Function to print the extreme nodes of // a given binary tree void printExtremeNodes(Node* root) {     // Stores height of binary tree     int h = height(root);        // Flag to mark the change in level     int flag = 0;        // To check if the extreme node of a     // particular level has been visited     int f = 0;        for (int i = 1; i <= h; i++) {         // If flag is zero then traverse from         // right to left at the given level and         // print the first node during the traversal         if (flag == 0) {             rightToLeft(root, i, f);             flag = 1;         }            // If flag is one then traverse from         // left to right at the given level and         // print the first node during the traversal         else if (flag == 1) {             leftToRight(root, i, f);             flag = 0;         }     }        return; }    // Driver code int main() {     Node* root = newNode(1);        root->left = newNode(2);     root->right = newNode(3);        root->left->left = newNode(4);     root->left->right = newNode(5);     root->right->right = newNode(7);        root->left->left->left = newNode(8);     root->left->left->right = newNode(9);     root->left->right->left = newNode(10);     root->left->right->right = newNode(11);     root->right->right->left = newNode(14);     root->right->right->right = newNode(15);        root->left->left->left->left = newNode(16);     root->left->left->left->right = newNode(17);     root->right->right->right->right = newNode(31);        printExtremeNodes(root);        return 0; }

## Java

 // Java program to print nodes of extreme corners  // of each level in alternate order  import java.util.*;    class GFG {        //INT class static class INT {     int a; }    // A binary tree node  static class Node  {      int data;      Node left, right;  };     // Utility function to allocate memory for a new node  static Node newNode(int data)  {      Node node = new Node();      node.data = data;      node.left = node.right = null;      return (node);  }     // Function that returns the height of the binary tree  static int height(Node root)  {      if (root == null)          return 0;         int lheight = height(root.left);      int rheight = height(root.right);         return Math.max(lheight, rheight) + 1;  }     // Function performs level order traversal from right to  // left and prints the first node during the traversal  static void rightToLeft(Node root, int level, INT f)  {      if (root == null)          return;         // Checks for the value of f so that      // only first node is printed during      // the traversal and no other node is printed      if (level == 1 && f.a == 0)      {          System.out.printf("%d ", root.data);          f.a = 1;      }         else if (level > 1)      {          rightToLeft(root.right, level - 1, f);          rightToLeft(root.left, level - 1, f);      }  }     // Function performs level order traversal from left to  // right and prints the first node during the traversal  static void leftToRight(Node root, int level, INT f)  {      if (root == null)          return;         // Checks for the value of f so that      // only first node is printed during      // the traversal and no other node is printed      if (level == 1 && f.a == 1)      {          System.out.printf("%d ", root.data);          f.a = 0;      }         else if (level > 1)     {          leftToRight(root.left, level - 1, f);          leftToRight(root.right, level - 1, f);      }  }     // Function to print the extreme nodes of  // a given binary tree  static void printExtremeNodes(Node root)  {      // Stores height of binary tree      int h = height(root);         // Flag to mark the change in level      int flag = 0;         // To check if the extreme node of a      // particular level has been visited      INT f=new INT();     f.a = 0;         for (int i = 1; i <= h; i++)      {          // If flag is zero then traverse from          // right to left at the given level and          // print the first node during the traversal          if (flag == 0)         {              rightToLeft(root, i, f);              flag = 1;          }             // If flag is one then traverse from          // left to right at the given level and          // print the first node during the traversal          else if (flag == 1)          {              leftToRight(root, i, f);              flag = 0;          }      }         return;  }     // Driver code  public static void main(String args[]) {      Node root = newNode(1);         root.left = newNode(2);      root.right = newNode(3);         root.left.left = newNode(4);      root.left.right = newNode(5);      root.right.right = newNode(7);         root.left.left.left = newNode(8);      root.left.left.right = newNode(9);      root.left.right.left = newNode(10);      root.left.right.right = newNode(11);      root.right.right.left = newNode(14);      root.right.right.right = newNode(15);         root.left.left.left.left = newNode(16);      root.left.left.left.right = newNode(17);      root.right.right.right.right = newNode(31);         printExtremeNodes(root);     }  }    // This code is contributed by Arnab Kundu

## C#

 // C# program to print nodes of extreme corners  // of each level in alternate order  using System;    class GFG {        //INT class public class INT {     public int a; }    // A binary tree node  public class Node  {      public int data;      public Node left, right;  };     // Utility function to allocate memory for a new node  static Node newNode(int data)  {      Node node = new Node();      node.data = data;      node.left = node.right = null;      return (node);  }     // Function that returns the height of the binary tree  static int height(Node root)  {      if (root == null)          return 0;         int lheight = height(root.left);      int rheight = height(root.right);         return Math.Max(lheight, rheight) + 1;  }     // Function performs level order traversal from right to  // left and prints the first node during the traversal  static void rightToLeft(Node root, int level, INT f)  {      if (root == null)          return;         // Checks for the value of f so that      // only first node is printed during      // the traversal and no other node is printed      if (level == 1 && f.a == 0)      {          Console.Write("{0} ", root.data);          f.a = 1;      }         else if (level > 1)      {          rightToLeft(root.right, level - 1, f);          rightToLeft(root.left, level - 1, f);      }  }     // Function performs level order traversal from left to  // right and prints the first node during the traversal  static void leftToRight(Node root, int level, INT f)  {      if (root == null)          return;         // Checks for the value of f so that      // only first node is printed during      // the traversal and no other node is printed      if (level == 1 && f.a == 1)      {          Console.Write("{0} ", root.data);          f.a = 0;      }         else if (level > 1)     {          leftToRight(root.left, level - 1, f);          leftToRight(root.right, level - 1, f);      }  }     // Function to print the extreme nodes of  // a given binary tree  static void printExtremeNodes(Node root)  {      // Stores height of binary tree      int h = height(root);         // Flag to mark the change in level      int flag = 0;         // To check if the extreme node of a      // particular level has been visited      INT f=new INT();     f.a = 0;         for (int i = 1; i <= h; i++)      {          // If flag is zero then traverse from          // right to left at the given level and          // print the first node during the traversal          if (flag == 0)         {              rightToLeft(root, i, f);              flag = 1;          }             // If flag is one then traverse from          // left to right at the given level and          // print the first node during the traversal          else if (flag == 1)          {              leftToRight(root, i, f);              flag = 0;          }      }         return;  }     // Driver code  public static void Main() {      Node root = newNode(1);         root.left = newNode(2);      root.right = newNode(3);         root.left.left = newNode(4);      root.left.right = newNode(5);      root.right.right = newNode(7);         root.left.left.left = newNode(8);      root.left.left.right = newNode(9);      root.left.right.left = newNode(10);      root.left.right.right = newNode(11);      root.right.right.left = newNode(14);      root.right.right.right = newNode(15);         root.left.left.left.left = newNode(16);      root.left.left.left.right = newNode(17);      root.right.right.right.right = newNode(31);         printExtremeNodes(root);     }  }    /* This code contributed by PrinciRaj1992 */

Output:

1 2 7 8 31

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