左神视频day04——题目七：反转单向和双向链表

1.反转单向链表

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

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

    public static Node reverseList(Node head) {
        Node pre = null;
        Node next = null;
        while (head != null) {
            next = head.next;
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }

leetcode 92 题：反转从位置 m 到 n 的链表

public class LC92 {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        ListNode res = new ListNode(0);
        res.next = head;
        ListNode node = res;

        //找到需要反转的那一段的上一个节点，即下标为 m-2 处，m-1 是反转段的开始下标
        for (int i = 0; i < m - 1; i++) {
            node = node.next;
        }

        //此时node.next即为反转段的起点
        ListNode start = node.next;
        ListNode pre = null;
        ListNode next = null;

        //反转 m 到 n 这段，即下标 m-1 到 n-1 这段
        for (int i = 0; i <= n - m; i++) {
            next = start.next;
            start.next = pre;
            pre = start;
            start = next;
        }

        //反转段的上一个节点指向反转后的头结点
        node.next = pre;

        //找到反转段最后一个节点
        while (pre.next != null) {
            pre = pre.next;
        }

        //使反转段的最后一个节点指向反转段后面的第一个节点，此时next指向着这个节点
        pre.next = next;

        return res.next;
    }
}

2.反转双向链表

• 与上面反转单链表的代码只有两处不同：①定义结点时加入last指针；②添加 head.last = next

	public static class DoubleNode {
        public int value;
        public DoubleNode next;
        public DoubleNode last; //与反转单链表的不同之处

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

    public static DoubleNode reverseList(DoubleNode head) {
        DoubleNode pre = null;
        DoubleNode next = null;
        while (head != null) {
            next = head.next;
            head.next = pre;
            head.last = next; //与反转单链表的不同之处
            pre = head;
            head = next;
        }
        return pre;
    }