Data Structure Linked List: Merge Sort for Linked Lists

http://www.geeksforgeeks.org/merge-sort-for-linked-list/

 1 #include <iostream>

 2 #include <vector>

 3 #include <algorithm>

 4 #include <queue>

 5 #include <stack>

 6 #include <string>

 7 #include <fstream>

 8 #include <map>

 9 #include <set>

10 using namespace std;

11 

12 struct node {

13     int data;

14     node *next;

15     node() : data(0), next(NULL) { }

16     node(int d) : data(d), next(NULL) { }

17 };

18 

19 void push(node* &head, int k) {

20     node *new_node = new node(k);

21     new_node->next = head;

22     head = new_node;

23 }

24 

25 void print(node* head) {

26     if (!head) return;

27     cout << head->data << " ";

28     print(head->next);

29 }

30 

31 void frontbacksplit(node *head, node *&a, node *&b) {

32     node *p, *q;

33     if (!head || !head->next) {

34         a = head;

35         b = NULL;

36         return;

37     }

38     p = head;

39     q = head->next;

40     while (q) {

41         q = q->next;

42         if (q) {

43             q = q->next;

44             p = p->next;

45         }

46     }

47     a = head;

48     b = p->next;

49     p->next = NULL;

50 }

51 

52 node *sortmerge(node *a, node *b) {

53     node *ans = NULL;

54     if (!a) return b;

55     if (!b) return a;

56     if (a->data < b->data) {

57         ans = a;

58         ans->next = sortmerge(a->next, b);

59     }

60     else {

61         ans = b;

62         ans->next = sortmerge(a, b->next);

63     }

64     return ans;

65 }

66 

67 void mergesort(node *&head) {

68     if (!head || !head->next) return;

69     node *a, *b;

70     node *h = head;

71     frontbacksplit(h, a, b);

72     mergesort(a);

73     mergesort(b);

74     head = sortmerge(a, b);

75 }

76 

77 int main() {

78     node *head = NULL;

79     push(head, 15);

80     push(head, 10);

81     push(head, 5);

82     push(head, 20);

83     push(head, 3);

84     push(head, 2);

85     mergesort(head);

86     print(head);

87     return 0;

88 }