单链表之o(1)删除指定节点(Java)

思路：
1、如果删除的节点不是尾节点，只需要把下一个节点的值复制到当前节点，然后当前节点指向下下个节点即可

2、如果删除的是尾节点，

• 链表只有一个元素，直接删除即可；
• 遍历找到尾节点的前一个节点，把此节点的next指向null即可

 

代码：

package com.datastructure.link;

/**
 * o(1) 时间内删除链表节点
 */
public class RemoveNodeInLink {

	static class Node {
		int value;
		Node next;
		public Node(int value) {
			this.value = value;
		}
		
		// 便于打印不能作用与循环链表
        @Override
        public String toString() {
            if (this.next == null) {
                return String.valueOf(this.value);
            }
            return this.value + "->" + this.next.toString();
        }
	}
	
	/**
	 * o(n)时间 删除节点 按照索引位置删除节点
	 * 
	 * 
	 * @return 返回新的链表(新链表，头结点引用)
	 * 
	 */
	public static Node removeNode(Node head, int index) {
		if (head == null || index < 0) {
			throw new RuntimeException("Invalid param");
		}
		
		if (index == 0) {
			return head.next;
		}
		
		Node preNode = head;
		Node curNode = head.next;
		
		int count = 1;
		while (curNode != null) {
			if (count == index) {
				preNode.next = curNode.next;
				break;
			}
			count++;
			preNode = preNode.next;
			curNode = curNode.next;
		}
		
		return head;
	}
	
	/**
	 * o(1) 删除指定元素 
	 */
	public static Node removeNode(Node head, Node toRemove) {
		if (head == null || toRemove == null) {
			return head;
		}
		// 非尾节点
		if (toRemove.next != null) {
			Node toRemoveNext = toRemove.next;
			toRemove.value = toRemoveNext.value;
			toRemove.next = toRemoveNext.next;
			return head;
		// 尾节点	
		}else {
			// 头节点即是尾节点
			if (head == toRemove) {
				return null;
			}
			// 找到尾节点的前一个节点
			Node tmp = head;
			// 或者
			// while (tmp.next != null && tmp.next.next != null)
			while (tmp.next != toRemove) {
				tmp = tmp.next;
			}
			tmp.next = null;
			return head;
		}
		
	}
	
    public static void main(String args[]) {
        
		Node head = createTestLinkedList(5, null);
	
        // 原链表
        System.out.println("origin link: " + head);

        // 删除第二个元素后
        System.out.println("remove third node: " + removeNode(head, 2));
        
        // 删除第二个元素后
        System.out.println("remove third node: " + removeNode(head, 2));
        
        
        Node toRemove = head.next.next;
        // 删除第三个元素
        System.out.println("remove third node: " + removeNode(head, toRemove));
        
        Node toRemove1 = head.next;
        // 删除第二个元素
        System.out.println("remove second node: " + removeNode(head, toRemove1));
        
        Node toRemoveHead = head;
        // 删除第一个元素
        System.out.println("remove first node: " + removeNode(head, toRemoveHead));
        
    }
    
    private static Node createTestLinkedList(int n, Node addNode) {
        Node head = new Node(0);
        Node curNode = head;
        for (int i = 1; i < n; i++) {
            curNode.next = new Node(i);
            curNode = curNode.next;
        }
        curNode.next = addNode;
        return head;
    }


}