【K 个一组翻转链表】模拟

leetcode 25. K 个一组翻转链表

假设当前需要反转的子链表为[curHead, curTail]
curDummy：当前需要反转的子链表的虚拟节点
curHead：当前需要反转的子链表的头节点
curTail：当前需要反转的子链表的尾节点

1. 找到尾节点curTail
2. 反转子链表[curHead, curTail]（反转子链表解法参考反转子链表题解2）
3. 更新curDummy、curHead
4. 重复1、2、3步，直至尾节点curTail为null（子链表不满足长度），头节点curHead为null（已经反转完所有的子链表）

题解1

小细节：反转子链表时，如果head==tail，子链表长度为1时，记得将子链表与原链表断开，否则 while(curDummy.next != null)会死循环。

反转子链表——递归代码

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode dummy = new ListNode(0, head);
        ListNode curHead = head, curTail = head, curDummy = dummy;
        while(curHead != null) {
            curTail = cut(curHead, k);
            if(curTail == null) {
                curDummy.next = curHead;
                break;
            }
            ListNode next = curTail.next;
            curDummy.next = reverse(curHead, curTail);
            while(curDummy.next != null) curDummy = curDummy.next;
            curHead = next;
        }
        return dummy.next;
    }

    public ListNode cut(ListNode head, int len) { // 返回尾节点
        ListNode tail = head;
        -- len;
        while(tail != null && len -- > 0) {
            tail = tail.next;
        }
        if(len > 0) return null;
        return tail;
    }

    public ListNode reverse(ListNode head, ListNode tail) {
        if(head == tail) {
            head.next = null;
            return head;
        }
        ListNode res = reverse(head.next, tail);
        head.next.next = head;
        head.next = null;
        return res;
    }
}

题解2

小细节：该种解法没有断开子链表，因此 while(curDummy.next != next)循环直至子链表的下一个节点结束。

反转子链表——空间复杂度O(1)代码

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode dummy = new ListNode(0, head);
        ListNode curDummy = dummy, curHead = head, curTail;
        while(curHead != null) {
            curTail = findTail(curHead, k);

            if(curTail == null) {
                curDummy.next = curHead;
                break;
            }
            ListNode next = curTail.next;
            curDummy.next = reverse(curDummy, curHead, curTail, next);
            while(curDummy.next != next) curDummy = curDummy.next;
            curHead = next;
        }
        return dummy.next;
    }
    public ListNode findTail(ListNode head, int len) {
        -- len;
        while(head != null && len -- > 0) {
            head = head.next;
        }
        if(len > 0) return null; // 此时子链表不满足长度len
        return head;
    }
    public ListNode reverse(ListNode dummy, ListNode head, ListNode tail, ListNode next) {
        ListNode suc = head.next;
        while(suc != next) {
            head.next = suc.next;
            suc.next = dummy.next;
            dummy.next = suc;
            if(head != next) suc = head.next;
        }
        return dummy.next;
    }
}