链表题目

一、翻转链表

/**
 * 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 reverseList(ListNode head) {
        ListNode prev = null, curr = head;
        while (curr != null) {
            ListNode next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
}

// 递归

class Solution {

    public ListNode reverseList(ListNode head) {

        if (head == null || head.next == null) {    // null 或 单节点

            return head;

        }

        ListNode p = reverseList(head.next);

        head.next.next = head;

        head.next = null;

        return p;

    }

}