q206_反转链表

public class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}
/**
 * 遍历直接反向修改next指针 o(n)
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre  = null;
        ListNode temp = head;
        while (temp != null) {
            ListNode t = temp.next;
            temp.next = pre;
            pre = temp;
            temp = t;
        }
        return pre;
    }
}

public class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}
/**
 * 递归法 o(n)
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode p = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return p;
    }
}