反转链表

[Aras-ax]

反转一个单链表。

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？

解答

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
// 迭代
var reverseList = function(head) {
    let last,
        next = head;

    while(next){
        next = head.next;
        head.next = last;
        last = head;
        head = next ? next : head;
    }

    return head;
};

// 递归
function reverseList(head) {
    if (head === null) {
        return null;
    }

    return reverse(head);
}

function reverse(head, last) {
    if (head) {
        let next = head.next;
        head.next = last;
        last = head;
        return reverse(next, last);
    }
    return last;
}