rt, 最近的面試題，面的時候敲的血崩，特此寫下部落格以作留念

如有錯誤，歡迎指正

class Node{

    int val;

    Node next;

}

import java.util.Random;

class Node {
    public int val;
    public Node next;
    public Node(int val) {
        this.val = val;
        this.next = null;
    }
}
public class Test {
    public static void main(String[] args) {
        Random random = new Random(47);
        Node head = new Node(10);
        Node p = head;
        for(int i = 0; i < 9; ++i) {
            p.next = new Node(random.nextInt(20));
            p = p.next;
        }
        System.out.print("歸併前：");
        for (Node i = head; i != null; i = i.next)
            System.out.print(i.val + " ");
        System.out.println();
        head = MergeSort(head, 10);

        System.out.print("歸併後：");
        for (Node i = head; i != null; i = i.next)
            System.out.print(i.val + " ");
        System.out.println();
    }

    //傳入連結串列頭和連結串列長度,返回新連結串列頭,對以head開始的長度為len的連結串列排序
    private static Node MergeSort(Node head, int len) {
        if (len > 1) {
            int m = len / 2;
            head = MergeSort(head, m);
            Node p = head;
            for (int i = 0; i < m - 1; ++i)
                p = p.next;
            p.next = MergeSort(p.next, len - m);
            head = merge(head, len, m);
        }
        return head;
    }

    //對以head開始的長度為len的連結串列合併, [1, m]和[m+1, len]的兩條連結串列
    private static Node merge(Node head, int len, int m) {
        Node p1 = head;
        Node p1Pre = null;
        Node p2 = head.next;
        Node p2Pre = head;
        for (int i = 0; i < m - 1; ++i) {
            p2 = p2.next;
            p2Pre = p2Pre.next;
        }
        int l = 1;
        int r = m + 1;
        while (l <= m && r <= len) {        //將連結串列歸併到p1鏈上
            if (p1.val <= p2.val) {
                p1Pre = p1;
                p1 = p1.next;
                ++l;
            } else {             //將p2指向的節點移到p1的前面
                p2Pre.next = p2.next;
                p2.next = p1;
                if (p1Pre == null) {
                    head = p2;
                    p1Pre = head;
                } else {
                    p1Pre.next = p2;
                    p1Pre = p2;
                }
                p2 = p2Pre.next;
                ++r;
            }
        }
        return head;
    }
}
 

雖然連結串列的歸併無需輔存，但連結串列的查詢增加了時間複雜度，而且寫的時候容易出錯，連結串列的歸併也並不好用