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Reverse tree path using Queue
• Difficulty Level : Hard
• Last Updated : 08 Nov, 2020

Given a tree and a node, the task is to reverse the path till the given Node and print the in-order traversal of the modified tree.
Examples:

```Input:
7
/   \
6     5
/ \   / \
4   3 2   1
Node = 4
Output: 7 6 3 4 2 5 1
The path from root to node 4 is 7 -> 6 -> 4
Reversing this path, the modified tree will be:
4
/   \
6     5
/ \   / \
7   3 2   1
whose in-order traversal is 7 6 3 4 2 5 1

Input:
7
/    \
6       5
/ \     / \
4  3     2  1
Node = 2
Output: 4 6 3 2 7 5 1

```

Approach:

• First store all the nodes on the given path in a queue.
• If the key is found then replace this node data with the front of queue data and pop the front.
• Keep on performing this operation in a recursive way up to the root and the path will be reversed in the original tree.
• Now, print the in-order traversal of the modified tree.

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach``#include ``using` `namespace` `std;` `// A Binary Tree Node``struct` `Node {``    ``int` `data;``    ``struct` `Node *left, *right;``};` `// Function to reverse the tree path``queue<``int``> reverseTreePathUtil(Node* root, ``int` `data,``                               ``queue<``int``> q1)``{``    ``queue<``int``> emptyQueue;` `    ``// If root is null then return``    ``// an empty queue``    ``if` `(root == NULL)``        ``return` `emptyQueue;` `    ``// If the node is found``    ``if` `(root->data == data) {` `        ``// Replace it with the queue's front``        ``q1.push(root->data);``        ``root->data = q1.front();``        ``q1.pop();``        ``return` `q1;``    ``}` `    ``// Push data into the queue for``    ``// storing data from start to end``    ``q1.push(root->data);` `    ``// If the returned queue is empty then``    ``// it means that the left sub-tree doesn't``    ``// contain the required node``    ``queue<``int``> left = reverseTreePathUtil(root->left,``                                          ``data, q1);` `    ``// If the returned queue is empty then``    ``// it means that the right sub-tree doesn't``    ``// contain the required node``    ``queue<``int``> right = reverseTreePathUtil(root->right,``                                           ``data, q1);` `    ``// If the required node is found``    ``// in the right sub-tree``    ``if` `(!right.empty()) {` `        ``// Replace with the queue's front``        ``root->data = right.front();``        ``right.pop();``        ``return` `right;``    ``}` `    ``// If the required node is found``    ``// in the right sub-tree``    ``if` `(!left.empty()) {` `        ``// Replace with the queue's front``        ``root->data = left.front();``        ``left.pop();``        ``return` `left;``    ``}` `    ``// Return emptyQueue if path``    ``// is not found``    ``return` `emptyQueue;``}` `// Function to call reverseTreePathUtil``// to reverse the tree path``void` `reverseTreePath(Node* root, ``int` `data)``{``    ``queue<``int``> q1;``    ``// reverse tree path``    ``reverseTreePathUtil(root, data, q1);``}` `// Function to print the in-order``// traversal of the tree``void` `inorder(Node* root)``{``    ``if` `(root != NULL) {``        ``inorder(root->left);``        ``cout << root->data << ``" "``;``        ``inorder(root->right);``    ``}``}` `// Utility function to create a new tree node``Node* newNode(``int` `data)``{``    ``Node* temp = ``new` `Node;``    ``temp->data = data;``    ``temp->left = temp->right = NULL;``    ``return` `temp;``}` `// Driver code``int` `main()``{``    ``Node* root = newNode(7);``    ``root->left = newNode(6);``    ``root->right = newNode(5);``    ``root->left->left = newNode(4);``    ``root->left->right = newNode(3);``    ``root->right->left = newNode(2);``    ``root->right->right = newNode(1);` `    ``int` `data = 4;` `    ``// Function call to reverse the path``    ``reverseTreePath(root, data);` `    ``// Print the in-order traversal``    ``// of the modified tree``    ``inorder(root);` `    ``return` `0;``}`

## Python3

 `# Python3 implementation of the``# above approach`` ` `# A Binary Tree Node``class` `Node:``    ` `    ``def` `__init__(``self``, data):``      ` `        ``self``.data ``=` `data``        ``self``.left ``=` `None``        ``self``.right ``=` `None`` ` `# Function to reverse the``# tree path``def` `reverseTreePathUtil(root,``                        ``data, q1):` `    ``emptyQueue ``=` `[]`` ` `    ``# If root is null then``    ``# return an empty queue``    ``if` `(root ``=``=` `None``):``        ``return` `emptyQueue;`` ` `    ``# If the node is found``    ``if` `(root.data ``=``=` `data):`` ` `        ``# Replace it with the``        ``# queue's front``        ``q1.append(root.data);``        ``root.data ``=` `q1[``0``]``        ``q1.pop(``0``);``        ``return` `q1;   `` ` `    ``# Push data into the``    ``# queue for storing``    ``# data from start to end``    ``q1.append(root.data);`` ` `    ``# If the returned queue``    ``# is empty then it means``    ``# that the left sub-tree``    ``# doesn't contain the``    ``# required node``    ``left ``=` `reverseTreePathUtil(root.left,``                               ``data, q1);`` ` `    ``# If the returned queue is empty``    ``# then it means that the right``    ``# sub-tree doesn't contain the``    ``# required node``    ``right ``=` `reverseTreePathUtil(root.right,``                                ``data, q1);`` ` `    ``# If the required node is found``    ``# in the right sub-tree``    ``if` `len``(right) !``=` `0``:`` ` `        ``# Replace with the queue's``        ``# front``        ``root.data ``=` `right[``0``]``        ``right.pop(``0``);``        ``return` `right;`` ` `    ``# If the required node``    ``# is found in the right``    ``# sub-tree``    ``if` `len``(left) !``=` `0``:`` ` `        ``# Replace with the``        ``# queue's front``        ``root.data ``=` `left[``0``]``        ``left.pop(``0``);``        ``return` `left;`` ` `    ``# Return emptyQueue``    ``# if path is not found``    ``return` `emptyQueue;`` ` `# Function to call reverseTreePathUtil``# to reverse the tree path``def` `reverseTreePath(root, data):` `    ``q1 ``=` `[]``    ` `    ``# reverse tree path``    ``reverseTreePathUtil(root,``                        ``data, q1);`` ` `# Function to print the in-order``# traversal of the tree``def` `inorder(root):` `    ``if` `(root !``=` `None``):``        ``inorder(root.left);``        ``print``(root.data,``              ``end ``=` `' '``)``        ``inorder(root.right);`` ` `# Utility function to create``# a new tree node``def` `newNode(data):` `    ``temp ``=` `Node(data)``    ``return` `temp;` `# Driver code``if` `__name__ ``=``=` `"__main__"``:``    ` `    ``root ``=` `newNode(``7``);``    ``root.left ``=` `newNode(``6``);``    ``root.right ``=` `newNode(``5``);``    ``root.left.left ``=` `newNode(``4``);``    ``root.left.right ``=` `newNode(``3``);``    ``root.right.left ``=` `newNode(``2``);``    ``root.right.right ``=` `newNode(``1``);`` ` `    ``data ``=` `4``;`` ` `    ``# Function call to reverse``    ``# the path``    ``reverseTreePath(root, data);`` ` `    ``# Print the in-order traversal``    ``# of the modified tree``    ``inorder(root);`` ` `# This code is contributed by Rutvik_56`
Output:
```7 6 3 4 2 5 1

```

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