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 C++ implementation of above idea.

// C++ program to flatten a multilevel linked list #include <bits/stdc++.h> using namespace std; // A Linked List Node struct Node { int data; struct Node *next; struct Node *down; }; // Flattens a multi-level linked list depth wise Node* 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 list void printFlattenNodes(Node* head) { while (head) { printf("%d ", head->data); head = head->next; } } // Utility function to create a new node Node* 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 code int 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; }

Output:

1 2 7 9 14 15 23 24 8 16 17 18 19 20 21 10 11 12 3 4

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