将单向链表按某值划分为左边小、中间相等、右边大的形式

【题目】
给定一个单链表的头节点head，节点的值类型是整型，再给定一个整数pivot。
实现一个调整链表的函数，将链表调整为左部分都是值小于pivot的节点，中间部分都是值等于pivot的节点，右部分都是值大于pivot的节点。

笔试：构建一个Node型数组，将单链表中的节点添加进数组中，在数组中进行操作，再把数组中的每一个Node窜起来

面试：用6个变量来实现，没有用额外的空间

package Algorithms;

/**
 * @author : zhang
 * @version : 1.0
 * @date : Create in 2021/8/10
 * @description :
 */

public class SmallerEqualBigger {

    public static void main(String[] args) {
        Node head1 = new Node(7);
        head1.next = new Node(9);
        head1.next.next = new Node(1);
        head1.next.next.next = new Node(8);
        head1.next.next.next.next = new Node(5);
        head1.next.next.next.next.next = new Node(2);
        head1.next.next.next.next.next.next = new Node(5);
        printLinkedList(head1);                      //Linked List: 7 9 1 8 5 2 5
        // head1 = listPartition1(head1, 5);
        head1 = listPartition2(head1, 5);
        printLinkedList(head1);                      //Linked List: 1 2 5 5 7 9 8 

    }

    public static class Node {
        public int value;
        public Node next;

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

    //方式1：笔试
    public static Node listPartition1(Node head, int pivot) {
        if (head == null) {
            return head;
        }
        Node cur = head;
        //统计链表长度
        int i = 0;
        while (cur != null) {
            i++;
            cur = cur.next;
        }
        //申请一个Node类型的数组
        Node[] nodeArr = new Node[i];
        cur = head;
        //for循环添加单链表中的Node进数组
        for (i = 0; i != nodeArr.length; i++) {
            nodeArr[i] = cur;
            cur = cur.next;
        }
        arrPartition(nodeArr, pivot);
        //将数组中的Node进行首尾串联
        for (i = 1; i != nodeArr.length; i++) {
            nodeArr[i - 1].next = nodeArr[i];
        }
        nodeArr[i - 1].next = null;
        return nodeArr[0];
    }

    //数组中进行Partition操作（< = >）
    public static void arrPartition(Node[] nodeArr, int pivot) {
        int small = -1;
        int big = nodeArr.length;  //big = 7
        int index = 0;
        while (index != big) {
            if (nodeArr[index].value < pivot) {
                swap(nodeArr, ++small, index++);
            } else if (nodeArr[index].value == pivot) {
                index++;
            } else {
                swap(nodeArr, --big, index);
            }
        }
    }

    public static void swap(Node[] nodeArr, int a, int b) {
        Node tmp = nodeArr[a];
        nodeArr[a] = nodeArr[b];
        nodeArr[b] = tmp;
    }

    //方式2：面试
    public static Node listPartition2(Node head, int pivot) {
        Node sH = null; // small head
        Node sT = null; // small tail
        Node eH = null; // equal head
        Node eT = null; // equal tail
        Node bH = null; // big head
        Node bT = null; // big tail
        Node next = null; // save next node
        // every node distributed to three lists
        while (head != null) {
            next = head.next;
            head.next = null;
            if (head.value < pivot) {
                if (sH == null) {
                    sH = head;
                    sT = head;
                } else {
                    sT.next = head;
                    sT = head;
                }
            } else if (head.value == pivot) {
                if (eH == null) {
                    eH = head;
                    eT = head;
                } else {
                    eT.next = head;
                    eT = head;
                }
            } else {
                if (bH == null) {
                    bH = head;
                    bT = head;
                } else {
                    bT.next = head;
                    bT = head;
                }
            }
            head = next;
        }
        // small and equal reconnect
        if (sT != null) { //如果有小于区域
            sT.next = eH;
            eT = eT == null ? sT : eT; //下一步，谁去连大于区域的头，谁就变成eT
        }
        // all reconnect
        if (eT != null) {
            eT.next = bH;
        }
        return sH != null ? sH : eH != null ? eH : bH;
    }

    public static void printLinkedList(Node node) {
        System.out.print("Linked List: ");
        while (node != null) {
            System.out.print(node.value + " ");
            node = node.next;
        }
        System.out.println();
    }
}

 数组中的数字按某值划分为左边小、中间相等、右边大的形式