【题目】反转部分单向链表

【题目】反转部分单向链表，时间复杂度O(N),空间复杂度O(1)

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

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

	public static Node reversePartNode(Node head, int from, int to) {

		Node node1 = head;
		Node pre = null;
		Node pos = null;
		int len = 0;
		while (node1 != null) {
			len++;
			pre = len == from - 1 ? node1 : pre;
			pos = len == to + 1 ? node1 : pos;
			node1 = node1.next;
		}
		if (from > to || from < 1 || to > len) {
			return head;
		}
		node1 = pre == null ? head : pre.next;
		Node node2 = node1.next;
		node1.next = pos;
		Node next = null;
		while (node2 != pos) {
			next = node2.next;
			node2.next = node1;
			node1 = node2;
			node2 = next;
		}
		if (pre != null) {
			pre.next = node1;
			return head;
		}
		return node1;
	}

}