Data Structure Linked List: Function to check if a singly linked list is palindrome

http://www.geeksforgeeks.org/function-to-check-if-a-singly-linked-list-is-palindrome/

这里的reverse可以reverse整个list，这样空间需求就是O(n)，不如这个网页写的O(1)的方法

 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     while (head) {

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

28         head = head->next;

29     }

30     cout << endl;

31 }

32 

33 void reverselist(node *&head) {

34     if (!head) return;

35     node *cur = head;

36     node *next = head->next;

37     if (!next) return;

38     reverselist(next);

39     cur->next->next = cur;

40     cur->next = NULL;

41     head = next;

42 }

43 

44 bool compare(node *first, node* second) {

45     while (first && second) {

46         if (first->data != second->data) return false;

47         first = first->next;

48         second = second->next;

49     }

50     return first == NULL && second == NULL;

51 }

52 

53 bool palin(node *head) {

54     if (!head || !head->next) return true;

55     node *p, *q, *pre;

56     p = q = pre = head;

57     node *midnode = NULL;

58     while (q && q->next) {

59         q = q->next->next;

60         pre = p;

61         p = p->next;

62     }

63     if (q) { //odd number

64         midnode = p;

65         p = p->next;

66     }

67     node *second = p;

68     pre->next = NULL;

69     reverselist(second);

70     bool ans = compare(head, second);

71     reverselist(second);

72     if (midnode) {

73         pre->next = midnode;

74         midnode->next = second;

75     }

76     else pre->next = second;

77     return ans;

78 }

79 

80 int main() {

81     node *head = NULL;

82     push(head, 1);

83     push(head, 2);

84     push(head, 3);

85     push(head, 3);

86     push(head, 2);

87     push(head, 1);

88     if (palin(head)) cout << "yes" << endl;

89     else cout << "no" << endl;

90     print(head);

91     return 0;

92 }

 recursive的法子很巧

 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     while (head) {

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

28         head = head->next;

29     }

30     cout << endl;

31 }

32 

33 bool palinhelp(node *&left, node *right) {

34     if (right == NULL) return true;

35     bool palin1 = palinhelp(left, right->next);

36     if (!palin1) return false;

37     bool palin2 = right->data == left->data;

38     left = left->next;

39     return palin2;

40 }

41 

42 bool palin(node *head) {

43     return palinhelp(head, head);

44 }

45 

46 int main() {

47     node *head = NULL;

48     push(head, 1);

49     push(head, 2);

50     push(head, 3);

51     push(head, 3);

52     push(head, 2);

53     push(head, 1);

54     if (palin(head)) cout << "yes" << endl;

55     else cout << "no" << endl;

56     print(head);

57     return 0;

58 }