算法题 — 链表反转

将单链表的链接顺序反转过来

• 例：输入：1->2->3->4->5
• 输出：5->4->3->2->1

使用两种方式解题

1 迭代

static class ListNode {
    int val;
    ListNode next;

    public ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }
}

public static ListNode reverseList(ListNode head) {
    if (head == null || head.next == null) {
        return head;
    }
    ListNode pre = null, cur = head, next;
    while (cur != null) {
        // 保存下一个节点
        next = cur.next;
        // 反转链表
        cur.next = pre;
        pre = cur;
        cur = next;
    }
    return pre;
}

public static void main(String[] args) {
   ListNode node5 = new ListNode(5, null);
   ListNode node4 = new ListNode(4, node5);
   ListNode node3 = new ListNode(3, node4);
   ListNode node2 = new ListNode(2, node3);
   ListNode node1 = new ListNode(1, node2);
   reverseList(node1)
}

2 递归

public static ListNode recursion(ListNode head) {
    if (head == null || head.next == null) {
        return head;
    }

    ListNode newHead = recursion(head.next);
    head.next.next = head;
    head.next = null;

    return newHead;
}