leetcode143-重排链表

重排链表

• 快慢指针+翻转链表

通过快慢指针找到中间节点，然后将后半段进行翻转，然后与前半段进行拼接。

class Solution {
    public void reorderList(ListNode head) {
        if(head == null || head.next == null)   return;
        ListNode fast = head.next, slow = head;
        while(fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode node = reverse(slow.next);
        slow.next = null;
        slow = head;
        while(slow != null && node != null){
            ListNode tmp = node;
            node = node.next;
            tmp.next = slow.next;
            slow.next = tmp;
            slow = slow.next.next;
        }
    }
    public ListNode reverse(ListNode head){
        if(head == null || head.next == null)   return head;
        ListNode prev = null;
        while(head != null){
            ListNode next = head.next;
            head.next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }
}