# Reverse a path in BST using queue

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 :

 `// CPP code to demonstrate insert ` `// operation in binary search tree ` `#include ` `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; ` `} `

 `# 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 `

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

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Improved By : PranchalKatiyar, rns111

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