關于鏈表的拓展 基本操作

一.通過X將鏈表排序；小的在前大的在后：
思考：遍歷整個鏈表，與X作比較，小的尾插在SMALL里，大的尾插在BIG 里；遍歷結束，判斷SMALL或者BIG是否為空，如果是則只返回另一個的第一個結點，

class ListNode {
    public int val;
    public ListNode next;

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

    public ListNode(int val) {
        this(val, null);
    }
}

public class LinkedListInterview {
    public ListNode separateByX(ListNode head, int x) {
        ListNode sHead = null;
        ListNode sEnd = null;
        ListNode bHead = null;
        ListNode bEnd = null;

        for (ListNode cur = head; cur != null; cur = cur.next) {
            if (cur.val < x) {
                if (sHead == null) {
                    sHead = cur;
                } else {
                    sEnd.next = cur;
                }
                sEnd = cur;
            } else {//≥
                if (bHead == null) {
                    bHead = cur;
                } else {
                    bEnd.next = cur;
                }
                bEnd = cur;
            }
        }

        if (sEnd == null) {
            return bHead;
        }

        sEnd.next = bHead;
        if (bEnd != null) {
            bEnd.next = null;
        }
        return sHead;
    }

    private static ListNode createTestList() {
        ListNode n1 = new ListNode(4);
        ListNode n2 = new ListNode(5);
        ListNode n3 = new ListNode(2);
        ListNode n4 = new ListNode(7);
        ListNode n5 = new ListNode(6);
        ListNode n6 = new ListNode(3);
        ListNode n7 = new ListNode(8);
        ListNode n8 = new ListNode(1);

        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;
        n6.next = n7;
        n7.next = n8;

        return n1;
    }

    // 三個重要節點
    // 1. 咱創建的測試鏈表是不是出問題了？
    // 2. 咱的分離程序是不是出問題了？
    // 3. 咱的打印程序是不是出問題了？
    private static void test() {
        // 4 5 2 7 6 3 8 1
        ListNode head = createTestList();
        ListNode result = new LinkedListInterview().separateByX(head, 5);
        // 4 2 3 1 5 7 6 8
        for (ListNode cur = result; cur != null; cur = cur.next) {
            System.out.println(cur.val);
        }
    }

    public static void main(String[] args) {//主函數，調用test子函數
        test();
    }
}