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| 1 | +/* |
| 2 | + * Given a linked list, reverse alternate nodes and append at the end |
| 3 | + * Examples: |
| 4 | + * Input List: 1->2->3->4->5->6 |
| 5 | + * Output List: 1->3->5->6->4->2 |
| 6 | + * |
| 7 | + * Input List: 12->14->16->18->20 |
| 8 | + * Output List: 12->16->20->18->14 |
| 9 | + */ |
| 10 | + |
| 11 | +#include <iostream> |
| 12 | + |
| 13 | +struct Node { |
| 14 | + int data; |
| 15 | + Node * next; |
| 16 | + Node(int d) : data{ d }, next{ nullptr } { } |
| 17 | +}; |
| 18 | + |
| 19 | +void insertAtHead( Node * & head, Node * node ) { |
| 20 | + if ( head != nullptr ) { |
| 21 | + node->next = head; |
| 22 | + } |
| 23 | + head = node; |
| 24 | +} |
| 25 | + |
| 26 | +void insert( Node * & head, int data ) { |
| 27 | + Node * newNode = new Node(data); |
| 28 | + if ( head == nullptr ) { |
| 29 | + head = newNode; |
| 30 | + } else { |
| 31 | + Node * curr = head; |
| 32 | + while( curr->next ) { |
| 33 | + curr = curr->next; |
| 34 | + } |
| 35 | + curr->next = newNode; |
| 36 | + } |
| 37 | +} |
| 38 | + |
| 39 | +void printList( Node * head ) { |
| 40 | + while( head ) { |
| 41 | + std::cout << head->data << "-->"; |
| 42 | + head = head->next; |
| 43 | + } |
| 44 | + std::cout << "NULL" << std::endl; |
| 45 | + } |
| 46 | + |
| 47 | + /* Algorithm function */ |
| 48 | + void rearrange( Node * & head ) { |
| 49 | + if ( head == nullptr ) { |
| 50 | + return; |
| 51 | + } |
| 52 | + Node * curr = head; |
| 53 | + Node * forw = head->next; |
| 54 | + if ( forw == nullptr ) { |
| 55 | + return; |
| 56 | + } |
| 57 | + Node * backHead = nullptr; |
| 58 | + while ( curr != nullptr && forw != nullptr ) { |
| 59 | + curr->next = forw->next; |
| 60 | + forw->next = nullptr; |
| 61 | + insertAtHead( backHead, forw ); |
| 62 | + curr = curr->next; |
| 63 | + if (curr != nullptr ) { |
| 64 | + forw = curr->next; |
| 65 | + } |
| 66 | + } |
| 67 | + //adding the backlist to mainlist again. |
| 68 | + curr = head; |
| 69 | + while ( curr->next != nullptr ) { |
| 70 | + curr = curr->next; |
| 71 | + } |
| 72 | + curr->next = backHead; |
| 73 | + } |
| 74 | + |
| 75 | + int main() { |
| 76 | + Node * head = nullptr; |
| 77 | + insert( head, 1 ); |
| 78 | + insert( head, 2 ); |
| 79 | + insert( head, 3 ); |
| 80 | + insert( head, 4 ); |
| 81 | + insert( head, 5 ); |
| 82 | + insert( head, 6 ); |
| 83 | + std::cout << "List before rearrange: "; |
| 84 | + printList( head ); |
| 85 | + rearrange( head ); |
| 86 | + std::cout << "List after rearrange: "; |
| 87 | + printList( head ); |
| 88 | + |
| 89 | + Node * head2 = nullptr; |
| 90 | + insert( head2, 12 ); |
| 91 | + insert( head2, 14 ); |
| 92 | + insert( head2, 16 ); |
| 93 | + insert( head2, 18 ); |
| 94 | + insert( head2, 20 ); |
| 95 | + std::cout << "List before rearrange: "; |
| 96 | + printList( head2 ); |
| 97 | + rearrange( head2 ); |
| 98 | + std::cout << "List after rearrange: "; |
| 99 | + printList( head2 ); |
| 100 | + |
| 101 | + return 0; |
| 102 | + } |
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