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; ` `} ` |

*chevron_right*

*filter_none*

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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