Flatten a binary tree into linked list | Set-2
Given a binary tree, flatten it into a linked list. After flattening, the left of each node should point to NULL and right should contain next node in level order.
Example:
Input: 1 / \ 2 5 / \ \ 3 4 6 Output: 1 \ 2 \ 3 \ 4 \ 5 \ 6 Input: 1 / \ 3 4 / 2 \ 5 Output: 1 \ 3 \ 4 \ 2 \ 5
Approach:
An approach using recursion has already been discussed in the previous post. A pre-order traversal of the binary tree using stack has been implied in this approach. In this traversal, every time a right child is pushed in the stack, the right child is made equal to the left child and left child is made equal to NULL. If the right child of the node becomes NULL, the stack is popped and the right child becomes the popped value from the stack. The above steps are repeated until the size of the stack is zero or root is NULL.
Below is the implementation of the above approach:
C++
// C++ program to flatten the linked // list using stack | set-2 #include <iostream> #include <stack> using namespace std; struct Node { int key; Node *left, *right; }; /* utility that allocates a new Node with the given key */ Node* newNode( int key) { Node* node = new Node; node->key = key; node->left = node->right = NULL; return (node); } // To find the inorder traversal void inorder( struct Node* root) { // base condition if (root == NULL) return ; inorder(root->left); cout << root->key << " " ; inorder(root->right); } // Function to convert binary tree into // linked list by altering the right node // and making left node point to NULL Node* solution(Node* A) { // Declare a stack stack<Node*> st; Node* ans = A; // Iterate till the stack is not empty // and till root is Null while (A != NULL || st.size() != 0) { // Check for NULL if (A->right != NULL) { st.push(A->right); } // Make the Right Left and // left NULL A->right = A->left; A->left = NULL; // Check for NULL if (A->right == NULL && st.size() != 0) { A->right = st.top(); st.pop(); } // Iterate A = A->right; } return ans; } // Driver Code int main() { /* 1 / \ 2 5 / \ \ 3 4 6 */ // Build the tree Node* root = newNode(1); root->left = newNode(2); root->right = newNode(5); root->left->left = newNode(3); root->left->right = newNode(4); root->right->right = newNode(6); // Call the function to // flatten the tree root = solution(root); cout << "The Inorder traversal after " "flattening binary tree " ; // call the function to print // inorder after flattening inorder(root); return 0; return 0; } |
Java
// Java program to flatten the linked // list using stack | set-2 import java.util.Stack; class GFG { static class Node { int key; Node left, right; } /* utility that allocates a new Node with the given key */ static Node newNode( int key) { Node node = new Node(); node.key = key; node.left = node.right = null ; return (node); } // To find the inorder traversal static void inorder(Node root) { // base condition if (root == null ) return ; inorder(root.left); System.out.print(root.key + " " ); inorder(root.right); } // Function to convert binary tree into // linked list by altering the right node // and making left node point to null static Node solution(Node A) { // Declare a stack Stack<Node> st = new Stack<>(); Node ans = A; // Iterate till the stack is not empty // and till root is Null while (A != null || st.size() != 0 ) { // Check for null if (A.right != null ) { st.push(A.right); } // Make the Right Left and // left null A.right = A.left; A.left = null ; // Check for null if (A.right == null && st.size() != 0 ) { A.right = st.peek(); st.pop(); } // Iterate A = A.right; } return ans; } // Driver Code public static void main(String[] args) { /* 1 / \ 2 5 / \ \ 3 4 6 */ // Build the tree Node root = newNode( 1 ); root.left = newNode( 2 ); root.right = newNode( 5 ); root.left.left = newNode( 3 ); root.left.right = newNode( 4 ); root.right.right = newNode( 6 ); // Call the function to // flatten the tree root = solution(root); System.out.print( "The Inorder traversal after " + "flattening binary tree " ); // call the function to print // inorder after flattening inorder(root); } } // This code has been contributed by 29AjayKumar |
Python3
# Python3 program to flatten the linked # list using stack | set-2 class Node: def __init__( self , key): self .key = key self .left = None self .right = None # Utility that allocates a new Node # with the given key def newNode(key): node = Node(key) node.key = key node.left = node.right = None return (node) # To find the inorder traversal def inorder(root): # Base condition if (root = = None ): return inorder(root.left) print (root.key, end = ' ' ) inorder(root.right) # Function to convert binary tree into # linked list by altering the right node # and making left node point to None def solution(A): # Declare a stack st = [] ans = A # Iterate till the stack is not empty # and till root is Null while (A ! = None or len (st) ! = 0 ): # Check for None if (A.right ! = None ): st.append(A.right) # Make the Right Left and # left None A.right = A.left A.left = None # Check for None if (A.right = = None and len (st) ! = 0 ): A.right = st[ - 1 ] st.pop() # Iterate A = A.right return ans # Driver Code if __name__ = = '__main__' : ''' 1 / \ 2 5 / \ \ 3 4 6 ''' # Build the tree root = newNode( 1 ) root.left = newNode( 2 ) root.right = newNode( 5 ) root.left.left = newNode( 3 ) root.left.right = newNode( 4 ) root.right.right = newNode( 6 ) # Call the function to # flatten the tree root = solution(root) print ( "The Inorder traversal after " "flattening binary tree " , end = '') # Call the function to print # inorder after flattening inorder(root) # This code is contributed by rutvik_56 |
C#
// C# program to flatten the linked // list using stack | set-2 using System; using System.Collections.Generic; class GFG { public class Node { public int key; public Node left, right; } /* utility that allocates a new Node with the given key */ static Node newNode( int key) { Node node = new Node(); node.key = key; node.left = node.right = null ; return (node); } // To find the inorder traversal static void inorder(Node root) { // base condition if (root == null ) return ; inorder(root.left); Console.Write(root.key + " " ); inorder(root.right); } // Function to convert binary tree into // linked list by altering the right node // and making left node point to null static Node solution(Node A) { // Declare a stack Stack<Node> st = new Stack<Node>(); Node ans = A; // Iterate till the stack is not empty // and till root is Null while (A != null || st.Count != 0) { // Check for null if (A.right != null ) { st.Push(A.right); } // Make the Right Left and // left null A.right = A.left; A.left = null ; // Check for null if (A.right == null && st.Count != 0) { A.right = st.Peek(); st.Pop(); } // Iterate A = A.right; } return ans; } // Driver Code public static void Main(String[] args) { /* 1 / \ 2 5 / \ \ 3 4 6 */ // Build the tree Node root = newNode(1); root.left = newNode(2); root.right = newNode(5); root.left.left = newNode(3); root.left.right = newNode(4); root.right.right = newNode(6); // Call the function to // flatten the tree root = solution(root); Console.Write( "The Inorder traversal after " + "flattening binary tree " ); // call the function to print // inorder after flattening inorder(root); } } // This code contributed by Rajput-Ji |
Javascript
<script> // Javascript program to flatten the linked // list using stack | set-2 class Node { constructor() { this .key = 0; this .left = null ; this .right = null ; } } /* utility that allocates a new Node with the given key */ function newNode(key) { var node = new Node(); node.key = key; node.left = node.right = null ; return (node); } // To find the inorder traversal function inorder( root) { // base condition if (root == null ) return ; inorder(root.left); document.write(root.key + " " ); inorder(root.right); } // Function to convert binary tree into // linked list by altering the right node // and making left node point to null function solution(A) { // Declare a stack var st = []; var ans = A; // Iterate till the stack is not empty // and till root is Null while (A != null || st.length != 0) { // Check for null if (A.right != null ) { st.push(A.right); } // Make the Right Left and // left null A.right = A.left; A.left = null ; // Check for null if (A.right == null && st.length != 0) { A.right = st[st.length-1]; st.pop(); } // Iterate A = A.right; } return ans; } // Driver Code /* 1 / \ 2 5 / \ \ 3 4 6 */ // Build the tree var root = newNode(1); root.left = newNode(2); root.right = newNode(5); root.left.left = newNode(3); root.left.right = newNode(4); root.right.right = newNode(6); // Call the function to // flatten the tree root = solution(root); document.write( "The Inorder traversal after " + "flattening binary tree " ); // call the function to print // inorder after flattening inorder(root); // This code is contributed by famously. </script> |
The Inorder traversal after flattening binary tree 1 2 3 4 5 6
Complexity Analysis:
- Time Complexity: O(N)
- Auxiliary Space: O(Log N)