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

最新推荐文章于 2021-09-02 23:56:01 发布

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最新推荐文章于 2021-09-02 23:56:01 发布

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原文链接：http://www.cnblogs.com/yingzhongwen/p/3655620.html

本文介绍了一种高效检查单链表是否为回文的方法，通过遍历链表找到中间节点并反转后半部分，再比较前后两部分来判断是否为回文。提供两种实现方式：一种使用额外空间，另一种采用递归方式。

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 }