NowCode 每日一题 剑指offer [3] 从尾到头打印链表

题目描述

输入一个链表，按链表从尾到头的顺序返回一个ArrayList。

题目分析：

方法1：

取出元素，放在ArrayList<>()中，然后再创建一个ArrayList<>(),倒序遍历放进去。 时间复杂度 O(2n) => O(n) 空间复杂度 O(n)

方法2：

取出元素，放在ArrayList<>()中,使用插入指定位置的add(index,value),方法 时间复杂度 O(n) 空间复杂度 O(n)

方法3：

很明显的先进先出，使用栈实现 时间复杂度 O(n) 空间复杂度 O(n)

代码实现

public class _03_PrintList {

    public static void main(String[] args) {
        SelfList selfList = new SelfList();
        selfList.add(new ListNode(12));
        selfList.add(new ListNode(13));
        selfList.add(new ListNode(14));
        selfList.add(new ListNode(15));
        selfList.add(new ListNode(16));
        ListNode root = selfList.getRoot();
        ArrayList res = printListFromTailToHead(root);
        ArrayList res2 = printListFromTailToHeadStack(root);
        ArrayList res3 = printListFromTailToHead2(root);
        for (Integer r : res) {
            System.out.print(r + " ");
        }

        System.out.println();
        for (Integer r : res2) {
            System.out.print(r + " ");
        }
        System.out.println();
        for (Integer r : res3) {
            System.out.print(r + " ");
        }
    }

    public static ArrayList printListFromTailToHeadStack(ListNode listNode) {
        if (listNode == null) {
            return new ArrayList();
        }
        ListNode temp = listNode;
        Stack stack = new Stack<>();
        while (temp != null) {
            stack.push(temp.val);
            temp = temp.next;
        }
        ArrayList res = new ArrayList<>();
        while (!stack.isEmpty() && stack.size() != 0) {
            res.add(stack.pop());
        }
        return res;
    }


public static ArrayList printListFromTailToHead2(ListNode listNode) {
    if (listNode == null) {
        return new ArrayList();
    }
    ArrayList res = new ArrayList<>();
    ListNode temp = listNode;
    while (temp != null) {
        res.add(0, temp.val);
        temp = temp.next;
    }
    return res;
}

public static ArrayList printListFromTailToHead(ListNode listNode) {
    if (listNode == null) {
        return new ArrayList();
    }
    ArrayList al = new ArrayList<>();
    ArrayList res = new ArrayList<>();
    ListNode temp = listNode;
    while (temp != null) {
        al.add(temp.val);
        temp = temp.next;
    }
    for (int i = al.size() - 1; i >= 0; i--) {
        res.add(al.get(i));
    }
    return res;
}
}

class SelfList {
    private ListNode root = new ListNode(0);

    public ListNode getRoot() {
        return root;
    }

    public void printListNode() {
        if (root == null || root.next == null) {
            return;
        }
        ListNode temp = root;
        while (temp.next != null) {
            System.out.println(temp.next);
            temp = temp.next;
        }
    }

    public void add(ListNode node) {
        ListNode temp = root;
        while (temp.next != null) {
            temp = temp.next;
        }
        temp.next = node;
    }
}

class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }

    @Override
    public String toString() {
        return "ListNode{" +
                "val=" + val +
                '}';
    }
}

测试

16 15 14 13 12 0 
16 15 14 13 12 0 
16 15 14 13 12 0