链表——回文链表（快慢指针,栈，反转链表）_回文链表 快慢指针

//判断链表是否是回文结构
public class IsPalindromeList {

	public static class Node {
		public int value;
		public Node next;

		public Node(int data) {
			this.value = data;
		}
	}

	// need n extra space
	//需要n的额外空间
	public static boolean isPalindrome1(Node head) {
		//堆结构
		Stack<Node> stack = new Stack<Node>();
		//新建cur指向头节点
		Node cur = head;
		//遍历将链表推进堆中
		while (cur != null) {
			stack.push(cur);
			cur = cur.next;
		}
		//遍历头节点依次与stack弹出的值作比较
		while (head != null) {
			if (head.value != stack.pop().value) {
				return false;
			}
			head = head.next;
		}
		return true;
	}

	// need n/2 extra space
	//需要n/2的额外空间
	public static boolean isPalindrome2(Node head) {
		if (head == null || head.next == null) {
			return true;
		}
		//right为慢指针
		//cur为快指针
		Node right = head.next;
		Node cur = head;
		while (cur.next != null && cur.next.next != null) {
			right = right.next;
			cur = cur.next.next;
		}
		Stack<Node> stack = new Stack<Node>();
		//当慢指针不为空，将当前至最后的节点都推入堆中
		while (right != null) {
			stack.push(right);
			right = right.next;
		}
		//比较
		while (!stack.isEmpty()) {
			if (head.value != stack.pop().value) {
				return false;
			}
			head = head.next;
		}
		return true;
	}

	// need O(1) extra space
	//只需要常数级别的额外空间
	public static boolean isPalindrome3(Node head) {
		if (head == null || head.next == null) {
			return true;
		}
		Node n1 = head;
		Node n2 = head;
		//找到中点n1
		while (n2.next != null && n2.next.next != null) { // find mid node
			n1 = n1.next; // n1 -> mid
			n2 = n2.next.next; // n2 -> end
		}
		//修改中点右边的节点指向，使之全部指向中点
		n2 = n1.next; // n2 -> right part first node
		n1.next = null; // mid.next -> null
		Node n3 = null;
		while (n2 != null) { // right part convert
			n3 = n2.next; // n3 -> save next node	n3指定为n2下一位
			n2.next = n1; // next of right node convert		n2下一位修改为n1
			n1 = n2; // n1 move		将n1修改为n2
			n2 = n3; // n2 move		将n2修改为n3
		}

		//从链表头尾两端遍历比较
		n3 = n1; // n3 -> save last node	n3修改为n1保存最后一个节点
		n2 = head;// n2 -> left first node		n2指定为第一个节点
		boolean res = true;
		while (n1 != null && n2 != null) { // check palindrome
			if (n1.value != n2.value) {
				res = false;
				break;
			}
			n1 = n1.next; // left to mid
			n2 = n2.next; // right to mid
		}
		n1 = n3.next;
		n3.next = null;
		//还原链表
		while (n1 != null) { // recover list
			n2 = n1.next;
			n1.next = n3;
			n3 = n1;
			n1 = n2;
		}
		return res;
	}

	public static void printLinkedList(Node node) {
		System.out.print("Linked List: ");
		while (node != null) {
			System.out.print(node.value + " ");
			node = node.next;
		}
		System.out.println();
	}

	public static void main(String[] args) {

		Node head = null;
		printLinkedList(head);
		System.out.print(isPalindrome1(head) + " | ");
		System.out.print(isPalindrome2(head) + " | ");
		System.out.println(isPalindrome3(head) + " | ");
		printLinkedList(head);
		System.out.println("=========================");

		head = new Node(1);
		printLinkedList(head);
		System.out.print(isPalindrome1(head) + " | ");
		System.out.print(isPalindrome2(head) + " | ");
		System.out.println(isPalindrome3(head) + " | ");
		printLinkedList(head);
		System.out.println("=========================");

		head = new Node(1);
		head.next = new Node(2);
		printLinkedList(head);
		System.out.print(isPalindrome1(head) + " | ");
		System.out.print(isPalindrome2(head) + " | ");
		System.out.println(isPalindrome3(head) + " | ");
		printLinkedList(head);
		System.out.println("=========================");

		head = new Node(1);
		head.next = new Node(1);
		printLinkedList(head);
		System.out.print(isPalindrome1(head) + " | ");
		System.out.print(isPalindrome2(head) + " | ");
		System.out.println(isPalindrome3(head) + " | ");
		printLinkedList(head);
		System.out.println("=========================");

		head = new Node(1);
		head.next = new Node(2);
		head.next.next = new Node(3);
		printLinkedList(head);
		System.out.print(isPalindrome1(head) + " | ");
		System.out.print(isPalindrome2(head) + " | ");
		System.out.println(isPalindrome3(head) + " | ");
		printLinkedList(head);
		System.out.println("=========================");

		head = new Node(1);
		head.next = new Node(2);
		head.next.next = new Node(1);
		printLinkedList(head);
		System.out.print(isPalindrome1(head) + " | ");
		System.out.print(isPalindrome2(head) + " | ");
		System.out.println(isPalindrome3(head) + " | ");
		printLinkedList(head);
		System.out.println("=========================");

		head = new Node(1);
		head.next = new Node(2);
		head.next.next = new Node(3);
		head.next.next.next = new Node(1);
		printLinkedList(head);
		System.out.print(isPalindrome1(head) + " | ");
		System.out.print(isPalindrome2(head) + " | ");
		System.out.println(isPalindrome3(head) + " | ");
		printLinkedList(head);
		System.out.println("=========================");

		head = new Node(1);
		head.next = new Node(2);
		head.next.next = new Node(2);
		head.next.next.next = new Node(1);
		printLinkedList(head);
		System.out.print(isPalindrome1(head) + " | ");
		System.out.print(isPalindrome2(head) + " | ");
		System.out.println(isPalindrome3(head) + " | ");
		printLinkedList(head);
		System.out.println("=========================");

		head = new Node(1);
		head.next = new Node(2);
		head.next.next = new Node(3);
		head.next.next.next = new Node(2);
		head.next.next.next.next = new Node(1);
		printLinkedList(head);
		System.out.print(isPalindrome1(head) + " | ");
		System.out.print(isPalindrome2(head) + " | ");
		System.out.println(isPalindrome3(head) + " | ");
		printLinkedList(head);
		System.out.println("=========================");

	}

}
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