LeetCode 206. 反转链表(Reverse Linked List) 16

206. 反转链表
206. Reverse Linked List

题目描述
反转一个单链表。

每日一算法2019/5/19Day 16LeetCode206. Reverse Linked List

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

进阶:
你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

Java 实现
ListNode 类

class ListNode {
    int val;
    ListNode next;

    ListNode(int x) {
        val = x;
    }

    @Override
    public String toString() {
        return val + "->" + next;
    }
}

Iterative Solution

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
            ListNode nextTemp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }
}

Recursive Solution

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;
    }
}

测试类

public class Test {
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        ListNode a = new ListNode(2);
        ListNode b = new ListNode(3);
        ListNode c = new ListNode(4);
        ListNode d = new ListNode(5);
        head.next = a;
        a.next = b;
        b.next = c;
        c.next = d;

        System.out.println(head);
        System.out.println(new Solution().reverseList(head));
    }
}

运行结果
Iterative Solution

1->2->3->4->5->null
5->4->3->2->1->null

Recursive Solution

1->2->3->4->5->null
5->4->3->2->1->null

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参考资料

LeetCode 206. 反转链表(Reverse Linked List) 16

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