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Reverse a path in BST using queue
  • Difficulty Level : Medium
  • Last Updated : 26 Jan, 2021

Given a binary search tree and a key, your task to reverse path of the binary tree.
Prerequisite : Reverse path of Binary tree

Examples : 

Input :       50
           /     \
          30      70
         /  \    /  \
       20   40  60   80 
k = 70
Output :
Inorder before reversal :
20 30 40 50 60 70 80 
Inorder after reversal :
20 30 40 70 60 50 80 

Input :       8
           /     \
          3       10
         /  \       \
       1    6         14
           /  \      /
          4    7    13
k = 13
Output :
Inorder before reversal :
1 3 4 6 7 8 10 13 14
Inorder after reversal :
1 3 4 6 7 13 14 8 10

Approach : 
Take a queue and push all the element till that given key at the end replace node key with queue front element till root, then print inorder of the tree.

Below is the implementation of above approach :  

C++




// C++ code to demonstrate insert
// operation in binary search tree
#include <bits/stdc++.h>
using namespace std;
 
struct node {
    int key;
    struct node *left, *right;
};
 
// A utility function to
// create a new BST node
struct node* newNode(int item)
{
    struct node* temp = new node;
    temp->key = item;
    temp->left = temp->right = NULL;
    return temp;
}
 
// A utility function to
// do inorder traversal of BST
void inorder(struct node* root)
{
    if (root != NULL) {
        inorder(root->left);
        cout << root->key << " ";
        inorder(root->right);
    }
}
 
// reverse tree path using queue
void reversePath(struct node** node,
            int& key, queue<int>& q1)
{
    /* If the tree is empty,
    return a new node */
    if (node == NULL)
        return;
 
    // If the node key equal
    // to key then
    if ((*node)->key == key)
    {
        // push current node key
        q1.push((*node)->key);
 
        // replace first node
        // with last element
        (*node)->key = q1.front();
 
        // remove first element
        q1.pop();
 
        // return
        return;
    }
     
    // if key smaller than node key then
    else if (key < (*node)->key)
    {
        // push node key into queue
        q1.push((*node)->key);
 
        // recusive call itself
        reversePath(&(*node)->left, key, q1);
 
        // replace queue front to node key
        (*node)->key = q1.front();
 
        // performe pop in queue
        q1.pop();
    }
     
    // if key greater than node key then
    else if (key > (*node)->key)
    {
        // push node key into queue
        q1.push((*node)->key);
 
        // recusive call itself
        reversePath(&(*node)->right, key, q1);
 
        // replace queue front to node key
        (*node)->key = q1.front();
 
        // performe pop in queue
        q1.pop();
    }
 
    // return
    return;
}
 
/* A utility function to insert
a new node with given key in BST */
struct node* insert(struct node* node,
                              int key)
{
    /* If the tree is empty,
    return a new node */
    if (node == NULL)
        return newNode(key);
 
    /* Otherwise, recur down the tree */
    if (key < node->key)
        node->left = insert(node->left, key);
    else if (key > node->key)
        node->right = insert(node->right, key);
 
    /* return the (unchanged) node pointer */
    return node;
}
 
// Driver Program to test above functions
int main()
{
    /* Let us create following BST
              50
           /     \
          30      70
         /  \    /  \
       20   40  60   80 */
    struct node* root = NULL;
    queue<int> q1;
 
    // reverse path till k
    int k = 80;
    root = insert(root, 50);
    insert(root, 30);
    insert(root, 20);
    insert(root, 40);
    insert(root, 70);
    insert(root, 60);
    insert(root, 80);
 
    cout << "Before Reverse :" << endl;
    // print inoder traversal of the BST
    inorder(root);
 
    cout << "\n";
 
    // reverse path till k
    reversePath(&root, k, q1);
     
    cout << "After Reverse :" << endl;
 
    // print inorder of reverse path tree
    inorder(root);
 
    return 0;
}


Java




// Java code to demonstrate insert
// operation in binary search tree
import java.util.*;
class GFG
{
  static class node
  {
    int key;
    node left, right;
  };
  static node root = null;
  static Queue<Integer> q1;
  static int k;
 
  // A utility function to
  // create a new BST node
  static node newNode(int item)
  {
    node temp = new node();
    temp.key = item;
    temp.left = temp.right = null;
    return temp;
  }
 
  // A utility function to
  // do inorder traversal of BST
  static void inorder(node root)
  {
    if (root != null)
    {
      inorder(root.left);
      System.out.print(root.key + " ");
      inorder(root.right);
    }
  }
 
  // reverse tree path using queue
  static void reversePath(node node)
  {
 
    /* If the tree is empty,
        return a new node */
    if (node == null)
      return;
 
    // If the node key equal
    // to key then
    if ((node).key == k)
    {
 
      // push current node key
      q1.add((node).key);
 
      // replace first node
      // with last element
      (node).key = q1.peek();
 
      // remove first element
      q1.remove();
 
      // return
      return;
    }
 
    // if key smaller than node key then
    else if (k < (node).key)
    {
 
      // push node key into queue
      q1.add((node).key);
 
      // recusive call itself
      reversePath((node).left);
 
      // replace queue front to node key
      (node).key = q1.peek();
 
      // performe pop in queue
      q1.remove();
    }
 
    // if key greater than node key then
    else if (k > (node).key)
    {
 
      // push node key into queue
      q1.add((node).key);
 
      // recusive call itself
      reversePath((node).right);
 
      // replace queue front to node key
      (node).key = q1.peek();
 
      // performe pop in queue
      q1.remove();
    }
 
    // return
    return;
  }
 
  /* A utility function to insert
    a new node with given key in BST */
  static node insert(node node, int key)
  {
 
    /* If the tree is empty,
        return a new node */
    if (node == null)
      return newNode(key);
 
    /* Otherwise, recur down the tree */
    if (key < node.key)
      node.left = insert(node.left, key);
    else if (key > node.key)
      node.right = insert(node.right, key);
 
    /* return the (unchanged) node pointer */
    return node;
  }
 
  // Driver code
  public static void main(String[] args)
  {
 
    /* Let us create following BST
                  50
               /     \
              30      70
             /  \    /  \
           20   40  60   80 */
    q1 = new LinkedList<>();
 
    // reverse path till k
    k = 80;
    root = insert(root, 50);
    root = insert(root, 30);
    root = insert(root, 20);
    root = insert(root, 40);
    root = insert(root, 70);
    root = insert(root, 60);
    root = insert(root, 80);
    System.out.print("Before Reverse :"
                     + "\n");
    // print inoder traversal of the BST
    inorder(root);
    System.out.print("\n");
 
    // reverse path till k
    reversePath(root);
    System.out.print("After Reverse :"
                     + "\n");
 
    // print inorder of reverse path tree
    inorder(root);
  }
}
 
// This code is contributed by gauravrajput1


Python3




# Python3 code to demonstrate insert
# operation in binary search tree
class Node:
 
    # Constructor to create a new node
    def __init__(self, data):
        self.key = data
        self.left = None
        self.right = None
 
# A utility function to
# do inorder traversal of BST
def inorder(root):
    if root != None:
        inorder(root.left)
        print(root.key, end = " ")
        inorder(root.right)
         
# reverse tree path using queue
def reversePath(node, key, q1):
     
    # If the tree is empty,
    # return a new node */
    if node == None:
        return
 
    # If the node key equal
    # to key then
    if node.key == key:
         
        # push current node key
        q1.insert(0, node.key)
 
        # replace first node
        # with last element
        node.key = q1[-1]
 
        # remove first element
        q1.pop()
 
        # return
        return
     
    # if key smaller than node key then
    elif key < node.key:
         
        # push node key into queue
        q1.insert(0, node.key)
 
        # recusive call itself
        reversePath(node.left, key, q1)
 
        # replace queue front to node key
        node.key = q1[-1]
 
        # performe pop in queue
        q1.pop()
     
    # if key greater than node key then
    elif (key > node.key):
         
        # push node key into queue
        q1.insert(0, node.key)
 
        # recusive call itself
        reversePath(node.right, key, q1)
 
        # replace queue front to node key
        node.key = q1[-1]
 
        # performe pop in queue
        q1.pop()
 
    # return
    return
     
# A utility function to insert
#a new node with given key in BST */
def insert(node, key):
     
    # If the tree is empty,
    # return a new node */
    if node == None:
        return Node(key)
 
    # Otherwise, recur down the tree */
    if key < node.key:
        node.left = insert(node.left, key)
    elif key > node.key:
        node.right = insert(node.right, key)
 
    # return the (unchanged) node pointer */
    return node
     
# Driver Code
if __name__ == '__main__':
     
    # Let us create following BST
    #             50
    #         /     \
    #         30     70
    #         / \ / \
    #     20 40 60 80 */
    root = None
    q1 = []
 
    # reverse path till k
    k = 80;
    root = insert(root, 50)
    insert(root, 30)
    insert(root, 20)
    insert(root, 40)
    insert(root, 70)
    insert(root, 60)
    insert(root, 80)
 
    print("Before Reverse :")
     
    # print inoder traversal of the BST
    inorder(root)
 
    # reverse path till k
    reversePath(root, k, q1)
    print()
    print("After Reverse :")
 
    # print inorder of reverse path tree
    inorder(root)    
     
# This code is contributed by PranchalK


C#




// C# code to demonstrate insert
// operation in binary search tree
using System;
using System.Collections.Generic;
 
class GFG{
 
public class node
{
    public int key;
    public node left, right;
};
 
static node root = null;
static Queue<int> q1;
static int k;
 
// A utility function to
// create a new BST node
static node newNode(int item)
{
    node temp = new node();
    temp.key = item;
    temp.left = temp.right = null;
    return temp;
}
 
// A utility function to
// do inorder traversal of BST
static void inorder(node root)
{
    if (root != null)
    {
        inorder(root.left);
        Console.Write(root.key + " ");
        inorder(root.right);
    }
}
 
// Reverse tree path using queue
static void reversePath(node node)
{
     
    // If the tree is empty,
    // return a new node
    if (node == null)
        return;
     
    // If the node key equal
    // to key then
    if ((node).key == k)
    {
         
        // push current node key
        q1.Enqueue((node).key);
         
        // replace first node
        // with last element
        (node).key = q1.Peek();
         
        // Remove first element
        q1.Dequeue();
         
        // Return
        return;
    }
     
    // If key smaller than node key then
    else if (k < (node).key)
    {
         
        // push node key into queue
        q1.Enqueue((node).key);
         
        // Recusive call itself
        reversePath((node).left);
         
        // Replace queue front to node key
        (node).key = q1.Peek();
         
        // Performe pop in queue
        q1.Dequeue();
    }
     
    // If key greater than node key then
    else if (k > (node).key)
    {
         
        // push node key into queue
        q1.Enqueue((node).key);
         
        // Recusive call itself
        reversePath((node).right);
         
        // Replace queue front to node key
        (node).key = q1.Peek();
         
        // Performe pop in queue
        q1.Dequeue();
    }
     
    // Return
    return;
}
 
// A utility function to insert
// a new node with given key in BST
static node insert(node node, int key)
{
     
    // If the tree is empty,
    // return a new node
    if (node == null)
        return newNode(key);
     
    // Otherwise, recur down the tree
    if (key < node.key)
        node.left = insert(node.left, key);
    else if (key > node.key)
        node.right = insert(node.right, key);
     
    // Return the (unchanged) node pointer
    return node;
}
 
// Driver code
public static void Main(String[] args)
{
     
    /* Let us create following BST
          50
       /     \
      30      70
     /  \    /  \
    20   40  60   80 */
    q1 = new Queue<int>();
     
    // Reverse path till k
    k = 80;
    root = insert(root, 50);
    root = insert(root, 30);
    root = insert(root, 20);
    root = insert(root, 40);
    root = insert(root, 70);
    root = insert(root, 60);
    root = insert(root, 80);
    Console.Write("Before Reverse :" + "\n");
     
    // Print inoder traversal of the BST
    inorder(root);
    Console.Write("\n");
     
    // Reverse path till k
    reversePath(root);
    Console.Write("After Reverse :" + "\n");
     
    // Print inorder of reverse path tree
    inorder(root);
}
}
 
// This code is contributed by gauravrajput1


Output: 

Before Reverse :
20 30 40 50 60 70 80 
After Reverse :
20 30 40 80 60 70 50

 

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