Lexi's Leetcode solutions

keep 3个pointer：

• prevStart：要reverse的section的前一个节点
• start：section第一个
• end：section最后一个

然后就每次reverse好了section，和prevStart连上就行了，然后prevStart 越过k个，接着做。这是个练习linked list的好题。

public ListNode reverseKGroup(ListNode head, int k) {
    ListNode dummy = new ListNode(0);
    dummy.next = head;
    ListNode prevStart = dummy;
    while (prevStart != null) {
        ListNode start = prevStart.next;
        ListNode end = prevStart;
        for (int i = 0; i < k; i++) {
            end = end.next;
            if (end == null)
                return dummy.next;
        }
        prevStart.next = reverse(start, end);
        for (int i = 0; i < k; i++) {
            prevStart = prevStart.next;
        }
    }
    return dummy.next;
}
private ListNode reverse(ListNode start, ListNode end) {
    ListNode prev = start;
    ListNode curr = start.next;// want to make curr point to prev
    ListNode afterEnd = end.next;
    while (curr != afterEnd) {
        ListNode next = curr.next;
        curr.next = prev;
        prev = curr;
        curr = next;
    }
    start.next = afterEnd;
    return end;
}