LC.P2487[从链表中移除节点]

方法一：单调栈

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeNodes(ListNode head) {
        ListNode dummy = new ListNode(0, head);
        Deque<ListNode> stack = new ArrayDeque<>();
        for (ListNode cur = head; cur != null; cur = cur.next) {
            while (!stack.isEmpty() && stack.peek().val < cur.val) {
                stack.pop();
            }
            if (stack.isEmpty()) {
                dummy.next = cur;
            } else {
                stack.peek().next = cur;
            }
            stack.push(cur);
        }
        return dummy.next;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$

方法二：递归

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeNodes(ListNode head) {
        if (head == null) return null;
        head.next = removeNodes(head.next);
        if (head.next != null && head.val < head.next.val) {
            return head.next;
        } else {
            return head;
        }
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$