将单向链表按某值划分为左边小，中间相等，右边大的形式

/**
 * 将单向链表按某值划分为左边小，中间相等，右边大的形式
 */
public class NodePartition {

    /**
     * 将链表分为 小于、等于、大于 的三个链表，然后连成一个链表
     *
     * @param head 链表
     * @param key  指定值
     * @return 新链表
     */
    public Node partition(Node head, int key) {
        if (head == null || head.next == null) {
            return head;
        }
        Node lessLeft = null;
        Node lessRight = null;
        Node equalLeft = null;
        Node equalRight = null;
        Node moreLeft = null;
        Node moreRight = null;
        Node next = null;
        while (head != null) {
            // 分离剩余节点
            next = head.next;
            // 分离头节点
            head.next = null;
            if (head.value < key) {
                if (lessLeft == null) {
                    lessLeft = head;
                    lessRight = head;
                } else {
                    lessRight.next = head;
                    lessRight = lessRight.next;
                }
            } else if (head.value == key) {
                if (equalLeft == null) {
                    equalLeft = head;
                    equalRight = head;
                } else {
                    equalRight.next = head;
                    equalRight = equalRight.next;
                }
            } else {
                if (moreLeft == null) {
                    moreLeft = head;
                    moreRight = head;
                } else {
                    moreRight.next = head;
                    moreRight = moreRight.next;
                }
            }
            // 将头节点指向剩余节点
            head = next;
        }
        // 小于区域的尾巴，连等于区域的头，等于区域的尾巴连大于区域的头
        if (lessRight != null) {
            lessRight.next = equalLeft;
            equalRight = equalRight == null ? lessRight : equalRight;
        }
        if (equalRight != null) {
            equalRight.next = moreLeft;
        }
        return lessLeft != null ? lessLeft : (equalLeft != null ? equalLeft : moreLeft);
    }

    /**
     * 把链表放在数组里，数组做partition，之后再把链表拼回去
     */
    // public Node partition(Node head, int key) {}

    /**
     * 链表结构
     */
    public static class Node {

        public int value;

        public Node next;

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

    }

}

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