-
Notifications
You must be signed in to change notification settings - Fork 1
/
LeetCode-234-Palindrome-Linked-List.java
118 lines (94 loc) · 2.87 KB
/
LeetCode-234-Palindrome-Linked-List.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
/*
LeetCode: https://leetcode.com/problems/palindrome-linked-list/
LintCode: http://www.lintcode.com/problem/palindrome-linked-list/
JiuZhang: http://www.jiuzhang.com/solutions/palindrome-linked-list/
ProgramCreek: http://www.programcreek.com/2014/07/leetcode-palindrome-linked-list-java/
Analysis:
1.Reverse second part and compare
Time O(n)
Space O(1)
2.Reverse
*/
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
// 1.Reverse
// public boolean isPalindrome(ListNode head) {
// if(head == null || head.next == null) return true;
// int len = 0;
// ListNode curr = head;
// while(curr != null){
// len++;
// curr = curr.next;
// }
// if(len == 2) return head.val == head.next.val;
// int lo = 0;
// if(len % 2 == 0){
// lo = len / 2 + 1;
// }else{
// lo = len / 2 + 2;
// }
// curr = head;
// int i = 1;
// while(i < lo){
// curr = curr.next;
// i++;
// }
// curr = reverse(curr);
// while(head != null && curr != null){
// if(head.val != curr.val) return false;
// head = head.next;
// curr = curr.next;
// }
// return true;
// }
// private ListNode reverse(ListNode head){
// ListNode dummy = new ListNode(-1);
// dummy.next = null;
// while(head != null){
// ListNode next = head.next;
// head.next = dummy.next;
// dummy.next = head;
// head = next;
// }
// return dummy.next;
// }
// 2.Reverse
public boolean isPalindrome(ListNode head) {
if(head == null || head.next == null) return true;
ListNode mid = findMiddle(head);
ListNode second = reverse(mid);
while(head != null && second != null){
if(head.val != second.val) return false;
head = head.next;
second = second.next;
}
return true;
}
private ListNode findMiddle(ListNode head){
ListNode slow = head;
ListNode fast = head.next;
while(fast != null && fast.next != null){
slow = slow.next;
fast = fast.next.next;
}
return slow;
}
private ListNode reverse(ListNode head){
ListNode dummy = new ListNode(-1);
dummy.next = null;
while(head != null){
ListNode next = head.next;
head.next = dummy.next;
dummy.next = head;
head = next;
}
return dummy.next;
}
}