234 Palindrome Linked List

Author: int32bit

README.md

@@ -129,6 +129,7 @@
 + [230 Kth Smallest Element in a BST(BST、中序遍历)](algorithms/KthSmallestElementinaBST)
 + [231 Power of Two(位运算，二进制1的个数)](algorithms/PowerofTwo)
 + [232 Implement Queue using Stacks(栈模拟队列)](algorithms/ImplementQueueusingStacks)
++ [234 Palindrome Linked List(链表逆转、回文链表)](algorithms/PalindromeLinkedList)
 + [235 Lowest Common Ancestor of a Binary Search Tree(LCA, 最低公共祖先,二叉搜索树)](algorithms/LowestCommonAncestorofaBinarySearchTree)
 + [236 Lowest Common Ancestor of a Binary Tree(LCA,最低公共祖先，二叉树](algorithms/LowestCommonAncestorofaBinaryTree)
 + [237 Delete Node in a Linked List(O(1)删除单链表节点)](algorithms/DeleteNodeinaLinkedList)

algorithms/PalindromeLinkedList/README.md

@@ -0,0 +1,64 @@
+## Palindrome Linked List
+
+Given a singly linked list, determine if it is a palindrome.
+
+Follow up:
+Could you do it in O(n) time and O(1) space?
+
+## Solution
+
+找到中间节点，然后原地逆转右半部分，比如左半部分和右半部分是否相等
+
+首先实现找到中间节点，使用快慢指针:
+
+```cpp
+ListNode *getMiddleNode(ListNode *head) {
+	if (head == nullptr)
+		return nullptr;
+	ListNode *slow = head, *fast = head;
+	while (fast && fast->next) {
+		fast = fast->next->next;
+		slow = slow->next;
+	}
+	return slow;
+}
+```
+
+逆转链表:
+
+```cpp
+ListNode *reverse(ListNode *head) {
+	if (head == nullptr || head->next == nullptr)
+		return head;
+	ListNode *prev = nullptr;
+	ListNode *p = head;
+	while (p) {
+		ListNode *q = p->next;
+		p->next = prev;
+		prev = p;
+		p = q;
+	}
+	return prev;
+}
+```
+
+最后结果:
+
+```cpp
+bool isPalindrome(ListNode *head) {
+	/* 空节点和单节点 */
+	if (head == nullptr || head->next == nullptr)
+		return true;
+	ListNode *mid = getMiddleNode(head);
+	ListNode *h1 = head;
+	ListNode *h2 = reverse(mid->next);
+	while(h1 && h2) {
+		if (h1->val != h2->val)
+			return false;
+		h1 = h1->next;
+		h2 = h2->next;
+	}
+	return true;
+
+}
+```

algorithms/PalindromeLinkedList/list.cpp

@@ -0,0 +1,131 @@
+#include <iostream>
+#include <cstdio>
+#include <vector>
+#include <algorithm>
+#include <string>
+using namespace std;
+#include <stdlib.h>
+struct ListNode {
+	int val;
+	ListNode *next;
+	ListNode(int x): val(x), next(nullptr) {}
+};
+class Solution {
+	public:
+		bool isPalindrome(ListNode *head) {
+			/* 空节点和单节点 */
+			if (head == nullptr || head->next == nullptr)
+				return true;
+			ListNode *mid = getMiddleNode(head);
+			ListNode *h1 = head;
+			ListNode *h2 = reverse(mid->next);
+			while(h1 && h2) {
+				if (h1->val != h2->val)
+					return false;
+				h1 = h1->next;
+				h2 = h2->next;
+			}
+			return true;
+
+		}
+	private:
+		ListNode *getMiddleNode(ListNode *head) {
+			if (head == nullptr)
+				return nullptr;
+			ListNode *slow = head, *fast = head;
+			while (fast && fast->next) {
+				fast = fast->next->next;
+				slow = slow->next;
+			}
+			return slow;
+		}
+		ListNode *reverse(ListNode *head) {
+			if (head == nullptr || head->next == nullptr)
+				return head;
+			ListNode *prev = nullptr;
+			ListNode *p = head;
+			while (p) {
+				ListNode *q = p->next;
+				p->next = prev;
+				prev = p;
+				p = q;
+			}
+			return prev;
+		}
+		int getLength(ListNode *head) {
+			int size = 0;
+			ListNode *p = head;
+			while(p) {
+				size++;
+				p = p->next;
+			}
+			return size;
+		}
+
+};
+int getLength(ListNode *head)
+{
+	int len = 0;
+	ListNode *p = head;
+	while (p) {
+		++len;
+		p = p->next;
+	}
+	return len;
+}
+void print(ListNode *head)
+{
+	if (head == nullptr) {
+		printf("NULL\n");
+		return;
+	}
+	struct ListNode *p = head;
+	while (p) {
+		printf("%d ", p->val);
+		p = p->next;
+	}
+	printf("\n");
+}
+ListNode * mk_list(int a[], int n)
+{
+	if (n < 1)
+		return nullptr;
+	ListNode *head = new ListNode(a[0]);
+	ListNode *p = head;
+	for (int i = 1; i < n; ++i) {
+		ListNode *q = new ListNode(a[i]);
+		p->next = q;
+		p = q;
+	}
+	return p;
+}
+ListNode * mk_list(const vector<int> &v) {
+	int n = v.size();
+	if (n < 1)
+		return nullptr;
+	ListNode *head = new ListNode(v[0]);
+	ListNode *p = head;
+	for (int i = 1; i < n; ++i) {
+		ListNode *q = new ListNode(v[i]);
+		p->next = q;
+		p = q;
+	}
+	return head;
+}
+
+void free_list(struct ListNode *head)
+{
+	struct ListNode *p = head;
+	while (p) {
+		struct ListNode *q = p->next;
+		delete p;
+		p = q;
+	}
+}
+int main(int argc, char **argv)
+{
+	Solution solution;
+	struct ListNode *head = mk_list({1,2,2,1});
+	cout << solution.isPalindrome(head) << endl;
+	return 0;
+}