6.从尾到头打印链表

题目

输入链表的第一个节点，从尾到头反过来打印出每个结点的值。

思路

1.使用栈 O(N)+O(N)

public ArrayList printListFromTailToHead(ListNode listNode) {
    Stack stack = new Stack<>();
    while (listNode != null) {
        stack.add(listNode.val);
        listNode = listNode.next;
    }
    ArrayList ret = new ArrayList<>();
    while (!stack.isEmpty())
        ret.add(stack.pop());
    return ret;
}

2.使用递归

public ArrayList printListFromTailToHead(ListNode listNode) {
    ArrayList ret = new ArrayList<>();
    if (listNode != null) {
        ret.addAll(printListFromTailToHead(listNode.next));
        ret.add(listNode.val);
    }
    return ret;
}

3.头插法 O(N)+O(1)

public ArrayList printListFromTailToHead(ListNode listNode) {
    // 头插法构建逆序链表
    ListNode head = new ListNode(-1);
    while (listNode != null) {
        ListNode memo = listNode.next;
        listNode.next = head.next;
        head.next = listNode;
        listNode = memo;
    }
    // 构建 ArrayList
    ArrayList ret = new ArrayList<>();
    head = head.next;
    while (head != null) {
        ret.add(head.val);
        head = head.next;
    }
    return ret;
}

4.使用Collections.reserve方法

public ArrayList printListFromTailToHead(ListNode listNode) {
    ArrayList ret = new ArrayList<>();
    while (listNode != null) {
        ret.add(listNode.val);
        listNode = listNode.next;
    }
    Collections.reverse(ret);
    return ret;
}