Flatten a multi-level linked list | Set 2 (Depth wise)
We have discussed flattening of a multi-level linked list where nodes have two pointers down and next. In the previous post, we flattened the linked list level wise. How to flatten a linked list when we always need to process down pointer before next at every node.
Input:
1 - 2 - 3 - 4
|
7 - 8 - 10 - 12
| | |
9 16 11
| |
14 17 - 18 - 19 - 20
| |
15 - 23 21
|
24
Output:
Linked List to be flattened to
1 - 2 - 7 - 9 - 14 - 15 - 23 - 24 - 8
- 16 - 17 - 18 - 19 - 20 - 21 - 10 -
11 - 12 - 3 - 4
Note : 9 appears before 8 (When we are
at a node, we process down pointer before
right pointer)
Source : Oracle Interview
If we take a closer look, we can notice that this problem is similar to tree to linked list conversion. We recursively flatten a linked list with following steps.
1) If node is NULL, return NULL.
2) Store next node of current node (used in step 4).
3) Recursively flatten down list. While flattening, keep track of last visited node, so that the next list can be linked after it.
4) Recursively flatten next list (we get the next list from pointer stored in step 2) and attach it after last visited node.
Below is the implementation of above idea.
C++
// C++ program to flatten a multilevel linked list#include <bits/stdc++.h>using namespace std;// A Linked List Nodestruct Node{ int data; struct Node *next; struct Node *down;};// Flattens a multi-level linked list depth wiseNode* flattenList(Node* node){ // Base case if (node == NULL) return NULL; // To keep track of last visited node // (NOTE: This is static) static Node *last; last = node; // Store next pointer Node *next = node->next; // If down list exists, process it first // Add down list as next of current node if (node->down) node->next = flattenList(node->down); // If next exists, add it after the next // of last added node if (next) last->next = flattenList(next); return node;}// Utility method to print a linked listvoid printFlattenNodes(Node* head){ while (head) { printf("%d ", head->data); head = head->next; } }// Utility function to create a new nodeNode* newNode(int new_data){ Node* new_node = new Node; new_node->data = new_data; new_node->next = new_node->down = NULL; return new_node;}// Driver codeint main(){ // Creating above example list Node* head = newNode(1); head->next = newNode(2); head->next->next = newNode(3); head->next->next->next = newNode(4); head->next->down = newNode(7); head->next->down->down = newNode(9); head->next->down->down->down = newNode(14); head->next->down->down->down->down = newNode(15); head->next->down->down->down->down->next = newNode(23); head->next->down->down->down->down->next->down = newNode(24); head->next->down->next = newNode(8); head->next->down->next->down = newNode(16); head->next->down->next->down->down = newNode(17); head->next->down->next->down->down->next = newNode(18); head->next->down->next->down->down->next->next = newNode(19); head->next->down->next->down->down->next->next->next = newNode(20); head->next->down->next->down->down->next->next->next->down = newNode(21); head->next->down->next->next = newNode(10); head->next->down->next->next->down = newNode(11); head->next->down->next->next->next = newNode(12); // Flatten list and print modified list head = flattenList(head); printFlattenNodes(head); return 0;} |
Java
// Java program to flatten a multilevel linked listpublic class FlattenList { static Node last; // Flattens a multi-level linked list depth wise public static Node flattenList(Node node) { if(node==null) return null; // To keep track of last visited node // (NOTE: This is static) last = node; // Store next pointer Node next = node.next; // If down list exists, process it first // Add down list as next of current node if(node.down!=null) node.next = flattenList(node.down); // If next exists, add it after the next // of last added node if(next!=null) last.next = flattenList(next); return node; } // Utility method to print a linked list public static void printFlattenNodes(Node head) { Node curr=head; while(curr!=null) { System.out.print(curr.data+" "); curr = curr.next; } } // Utility function to create a new node public static Node push(int newData) { Node newNode = new Node(newData); newNode.next =null; newNode.down = null; return newNode; } public static void main(String args[]) { Node head=new Node(1); head.next = new Node(2); head.next.next = new Node(3); head.next.next.next = new Node(4); head.next.down = new Node(7); head.next.down.down = new Node(9); head.next.down.down.down = new Node(14); head.next.down.down.down.down= new Node(15); head.next.down.down.down.down.next= new Node(23); head.next.down.down.down.down.next.down = new Node(24); head.next.down.next = new Node(8); head.next.down.next.down = new Node(16); head.next.down.next.down.down= new Node(17); head.next.down.next.down.down.next= new Node(18); head.next.down.next.down.down.next.next= new Node(19); head.next.down.next.down.down.next.next.next = new Node(20); head.next.down.next.down.down.next.next.next.down = new Node(21); head.next.down.next.next = new Node(10); head.next.down.next.next.down = new Node(11); head.next.down.next.next.next = new Node(12); head = flattenList(head); printFlattenNodes(head); }}//Node of Multi-level Linked Listclass Node{ int data; Node next,down; Node(int data) { this.data=data; next=null; down=null; }}//This code is contributed by Gaurav Tiwari |
C#
// C# program to flatten a multilevel linked listusing System;class FlattenList { static Node last; // Flattens a multi-level linked list depth wise public static Node flattenList(Node node) { if(node == null) return null; // To keep track of last visited node // (NOTE: This is static) last = node; // Store next pointer Node next = node.next; // If down list exists, process it first // Add down list as next of current node if(node.down != null) node.next = flattenList(node.down); // If next exists, add it after the next // of last added node if(next != null) last.next = flattenList(next); return node; } // Utility method to print a linked list public static void printFlattenNodes(Node head) { Node curr = head; while(curr != null) { Console.Write(curr.data + " "); curr = curr.next; } } // Utility function to create a new node public static Node push(int newData) { Node newNode = new Node(newData); newNode.next =null; newNode.down = null; return newNode; } // Driver code public static void Main() { Node head = new Node(1); head.next = new Node(2); head.next.next = new Node(3); head.next.next.next = new Node(4); head.next.down = new Node(7); head.next.down.down = new Node(9); head.next.down.down.down = new Node(14); head.next.down.down.down.down= new Node(15); head.next.down.down.down.down.next= new Node(23); head.next.down.down.down.down.next.down = new Node(24); head.next.down.next = new Node(8); head.next.down.next.down = new Node(16); head.next.down.next.down.down= new Node(17); head.next.down.next.down.down.next= new Node(18); head.next.down.next.down.down.next.next= new Node(19); head.next.down.next.down.down.next.next.next = new Node(20); head.next.down.next.down.down.next.next.next.down = new Node(21); head.next.down.next.next = new Node(10); head.next.down.next.next.down = new Node(11); head.next.down.next.next.next = new Node(12); head = flattenList(head); printFlattenNodes(head); }}// Node of Multi-level Linked Listpublic class Node{ public int data; public Node next,down; public Node(int data) { this.data = data; next = null; down = null; }}/* This code is contributed PrinciRaj1992 */ |
Output:
1 2 7 9 14 15 23 24 8 16 17 18 19 20 21 10 11 12 3 4
Alternate implementation using stack data structure
C++
Node* flattenList2(Node* head){ Node* headcop = head; stack<Node*> save; save.push(head); Node* prev = NULL; while (!save.empty()) { Node* temp = save.top(); save.pop(); if (temp->next) save.push(temp->next); if (temp->down) save.push(temp->down); if (prev != NULL) prev->next = temp; prev = temp; } return headcop;} |
Java
Node flattenList2(Node head){ Node headcop = head; Stack<Node> save = new Stack<>(); save.push(head); Node prev = null; while (!save.isEmpty()) { Node temp = save.peek(); save.pop(); if (temp.next) save.push(temp.next); if (temp.down) save.push(temp.down); if (prev != null) prev.next = temp; prev = temp; } return headcop;}// This code contributed by aashish1995 |
Python3
def flattenList2(head): headcop = head save = [] save.append(head) prev = None while (len(save) != 0): temp = save[-1] save.pop() if (temp.next): save.append(temp.next) if (temp.down): save.append(temp.down) if (prev != None): prev.next = temp prev = temp return headcop# This code is contributed by rutvik_56 |
C#
Node flattenList2(Node head){ Node headcop = head; Stack<Node> save = new Stack<Node>(); save.Push(head); Node prev = null; while (!save.Count != 0) { Node temp = save.Peek(); save.Pop(); if (temp.next) save.Push(temp.next); if (temp.down) save.Push(temp.down); if (prev != null) prev.next = temp; prev = temp; } return headcop;}// This code is contributed by aashish1995 |
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