Offer.P6[从尾到头打印链表]

方法一：递归

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {

    List<Integer> list;

    public int[] reversePrint(ListNode head) {
        list = new ArrayList<>();
        reverse(head);
        int n = list.size();
        int[] ans = new int[n];
        for (int i = 0; i < n; ++i) {
            ans[i] = list.get(i);
        }
        return ans;
    }

    private void reverse(ListNode head) {
        if (head == null) return;
        reverse(head.next);
        list.add(head.val);
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$

方法二：栈

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {

    public int[] reversePrint(ListNode head) {
        Deque<Integer> stack = new ArrayDeque<>();
        while (head != null) {
            stack.push(head.val);
            head = head.next;
        }
        int n = stack.size();
        int[] ans = new int[n];
        for (int i = 0; i < n; ++i) {
            ans[i] = stack.pop();
        }
        return ans;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$