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

 #include <iostream>
 #include <vector>
 #include <algorithm>
 #include <queue>
 #include <stack>
 #include <string>
 #include <fstream>
 #include <map>
 #include <set>
 using namespace std;

 struct node {
     int data;
     node *next;
     node() : data(), next(NULL) { }
     node(int d) : data(d), next(NULL) { }
 };

 void push(node* &head, int k) {
     node *new_node = new node(k);
     new_node->next = head;
     head = new_node;
 }

 void print(node* head) {
     while (head) {
         cout << head->data << " ";
         head = head->next;
     }
     cout << endl;
 }

 void reverselist(node *&head) {
     if (!head) return;
     node *cur = head;
     node *next = head->next;
     if (!next) return;
     reverselist(next);
     cur->next->next = cur;
     cur->next = NULL;
     head = next;
 }

 bool compare(node *first, node* second) {
     while (first && second) {
         if (first->data != second->data) return false;
         first = first->next;
         second = second->next;
     }
     return first == NULL && second == NULL;
 }

 bool palin(node *head) {
     if (!head || !head->next) return true;
     node *p, *q, *pre;
     p = q = pre = head;
     node *midnode = NULL;
     while (q && q->next) {
         q = q->next->next;
         pre = p;
         p = p->next;
     }
     if (q) { //odd number
         midnode = p;
         p = p->next;
     }
     node *second = p;
     pre->next = NULL;
     reverselist(second);
     bool ans = compare(head, second);
     reverselist(second);
     if (midnode) {
         pre->next = midnode;
         midnode->next = second;
     }
     else pre->next = second;
     return ans;
 }

 int main() {
     node *head = NULL;
     push(head, );
     push(head, );
     push(head, );
     push(head, );
     push(head, );
     push(head, );
     if (palin(head)) cout << "yes" << endl;
     else cout << "no" << endl;
     print(head);
     return ;
 }

recursive的法子很巧

 #include <iostream>
 #include <vector>
 #include <algorithm>
 #include <queue>
 #include <stack>
 #include <string>
 #include <fstream>
 #include <map>
 #include <set>
 using namespace std;

 struct node {
     int data;
     node *next;
     node() : data(), next(NULL) { }
     node(int d) : data(d), next(NULL) { }
 };

 void push(node* &head, int k) {
     node *new_node = new node(k);
     new_node->next = head;
     head = new_node;
 }

 void print(node* head) {
     while (head) {
         cout << head->data << " ";
         head = head->next;
     }
     cout << endl;
 }

 bool palinhelp(node *&left, node *right) {
     if (right == NULL) return true;
     bool palin1 = palinhelp(left, right->next);
     if (!palin1) return false;
     bool palin2 = right->data == left->data;
     left = left->next;
     return palin2;
 }

 bool palin(node *head) {
     return palinhelp(head, head);
 }

 int main() {
     node *head = NULL;
     push(head, );
     push(head, );
     push(head, );
     push(head, );
     push(head, );
     push(head, );
     if (palin(head)) cout << "yes" << endl;
     else cout << "no" << endl;
     print(head);
     return ;
 }