Question

我试图从以下代码中了解为什么头部会被退回。我想它返回null。有人可以解释一下吗？

public ListNode removeNthFromEnd(ListNode head, int n) {
    if(head == null)
        return null;

    ListNode fast = head;
    ListNode slow = head;

    for(int i=0; i<n; i++){
        fast = fast.next;
    }

    //if remove the first node
    if(fast == null){
        head = head.next;
        return head;
    }

    while(fast.next != null){
        fast = fast.next;
        slow = slow.next;
    }

    slow.next = slow.next.next;

    return head;
}

Answer 1

我认为有一种更好的方法可以从列表末尾删除第n个元素。您可以使用堆栈。将元素放入堆栈中，直至到达末尾。然后，从头开始弹出直到第n个元素。然后，弹出元素的下一个（nth）是下一个。同样，堆栈中的第n个元素的前一个也会窥视。您所需要做的就是将下一个设置为下一个。您需要考虑删除列表的开头和结尾作为边缘情况。另外，您需要考虑空白列表。希望对您有所帮助。

import java.util.Stack;

public class RemoveNthNodeFromLL {

    public static class ListNode {
        int val;
        ListNode next;

        ListNode() {
        }

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

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

    public ListNode removeNthFromEnd(ListNode head, int n) {
        if (head == null || (head.next == null)) return null;
        Stack<ListNode> s = new Stack<>();
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
            prev = curr;
            s.push(prev);
            curr = curr.next;
        }
        for (int i = 1; i <= n - 1; i++) {
            s.pop();
        }
        ListNode toBeRemoved = s.pop();
        ListNode next = toBeRemoved.next;
        if (s.isEmpty()) {
            // Removing the head
            head = head.next;
        } else {
            prev = s.peek();
            prev.next = next;
        }
        return head;
    }

    public static void main(String[] args) {
        RemoveNthNodeFromLL r = new RemoveNthNodeFromLL();
        ListNode head = new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4, new ListNode(5)))));
        System.out.println(r.removeNthFromEnd(head, 2));
    }

}

从列表末尾删除第N个节点

1 个答案: