Print all nodes between two given levels in Binary Tree in C++

In this problem, we are given a binary tree and two levels in the tree (upper and lower) and we have to print all nodes between upper and lower levels of the tree.

The binary tree is a special tree whose each node has at max two nodes (one/two/none).

Let’s take an example to understand the problem −

upper − 1

lower − 3

Output −

6
3 9
7 4 8 10

To solve this problem, we have to print nodes of the tree at a given level. We will call a recursive function using a loop from the upper to lower level in the tree.

This algorithm is simple but is more complex of order n².

A more effective solution is doing inorder traversal and using a queue. And print nodes within the given upper and lower levels.

Program to implement our solution −

Example

 Live Demo

#include <iostream>
#include <queue>
using namespace std;
struct Node{
   int key;
   struct Node* left, *right;
};
void printNodesAtLevel(Node* root, int low, int high){
   queue <Node *> Q;
   Node *marker = new Node;
   int level = 1;
   Q.push(root);
   Q.push(marker);
   while (Q.empty() == false){
      Node *n = Q.front();
      Q.pop();
      if (n == marker){
         cout << endl;
         level++;
         if (Q.empty() == true || level > high) break;
         Q.push(marker);
         continue;
      }
      if (level >= low)
         cout<<n->key<<" ";
      if (n->left != NULL) Q.push(n->left);
      if (n->right != NULL) Q.push(n->right);
   }
}
Node* insertNode(int key){
   Node* temp = new Node;
   temp->key = key;
   temp->left = temp->right = NULL;
   return (temp);
}
int main() {
   struct Node *root = insertNode(6);
   root->left = insertNode(3);
   root->right = insertNode(9);
   root->left->left = insertNode(7);
   root->left->right = insertNode(4);
   root->left->right->left = insertNode(8);
   root->left->right->right = insertNode(10);
   root->left->right->right->left = insertNode(5);
   root->left->right->right->right = insertNode(1);
   root->left->right->left->left = insertNode(14);
   root->left->right->left->right = insertNode(26);
   int upper = 3;
   int lower = 1;
   cout << "Level wise Nodes between level "<<lower<<" and "<<upper<<" are \n";
   printNodesAtLevel(root, lower, upper);
   return 0;
}

Output

Level wise Nodes between level 1 and 3 are
6
3 9
7 4