Singly linked list reversal problem

1. Problem description

Reverse a singly linked list, such as 1-->2-->3-->4-->5-->null

The result after flipping is 5-->4-->3-->2-->1-->null

2. Solution

A traverse the linked list

Split the linked list into an array and then assemble the array into a linked list in reverse order

This method is simple to implement and does not require too many pointers. The disadvantage is that it requires n units of space

B pointer reverse order

Three pointers point to the current node (p), the next node of the current node (q), and the next node of q (r). The existence of r is to modify the next pointer of q to point to p, so that the next value of q will not be lost. The source code is as follows.

public class Node {

    public int value;
    public Node next;

    public Node(int value) {
        this.value = value;
    }

    public boolean hasNext() {
        return next != null;
    }
}

private static Node reverse(Node node) {
        Node p = node;
        Node q = p.next;
        Node r;

        p.next = null;
        while (q != null) {
            r = q.next;
            q.next = p;
            p = q;
            q = r;
        }
        return p;
    }

C recursion

First, treat the part D after the first node as a whole, and think that D is already in reverse order. What we need to do now is to reverse the order of the linked list composed of the first node and D node;

Secondly, it is necessary to reverse the order inside the D node and use the same thinking for processing;

Again, the recursive exit, when the next of the D node is empty, it means that the D node is the last node, just return directly.

The source code is as follows.

/**
     *
     * @param node 链表头结点
     * @param pre 头结点逆序指向的节点
     * @return
     */
    private static Node reverseRecursion(Node node, Node pre) {
        Node head = node;
        Node pnext = head.next;
        head.next = pre;
        if (pnext == null) {
            return head;
        }
        return reverseRecursion(pnext, head);
    }

the above

Java simple realization of singly linked list reversal