手写 skip list，看看自己最近有多脑残

lab-09/src/main/java/cn/iocoder/springboot/labs/lab09/search/SkipList.java

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+package cn.iocoder.springboot.labs.lab09.search;
+
+import java.util.Random;
+
+/**
+ * 跳表的一种实现方法。
+ * 跳表中存储的是正整数，并且存储的是不重复的。
+ *
+ * From https://github.com/wangzheng0822/algo/blob/master/java/17_skiplist/SkipList.java
+ *
+ * Author：ZHENG
+ */
+@SuppressWarnings("Duplicates")
+public class SkipList {
+
+    private static final int MAX_LEVEL = 16;
+
+    private int levelCount = 1;
+
+    private Node head = new Node();  // 带头链表
+
+    private Random r = new Random();
+
+    public Node find(int value) {
+        Node p = head;
+        for (int i = levelCount - 1; i >= 0; --i) {
+            while (p.forwards[i] != null && p.forwards[i].data < value) {
+                p = p.forwards[i];
+            }
+        }
+
+        if (p.forwards[0] != null && p.forwards[0].data == value) {
+            return p.forwards[0];
+        } else {
+            return null;
+        }
+    }
+
+    public void insert(int value) {
+        int level = randomLevel();
+        Node newNode = new Node();
+        newNode.data = value;
+        newNode.maxLevel = level;
+        Node[] update = new Node[level];
+        for (int i = 0; i < level; ++i) {
+            update[i] = head;
+        }
+
+        // record every level largest value which smaller than insert value in update[]
+        Node p = head;
+        for (int i = level - 1; i >= 0; --i) {
+            while (p.forwards[i] != null && p.forwards[i].data < value) {
+                p = p.forwards[i];
+            }
+            update[i] = p;// use update save node in search path
+        }
+
+        // in search path node next node become new node forwords(next)
+        for (int i = 0; i < level; ++i) {
+            newNode.forwards[i] = update[i].forwards[i];
+            update[i].forwards[i] = newNode;
+        }
+
+        // update node hight
+        if (levelCount < level) levelCount = level;
+    }
+
+    public void delete(int value) {
+        Node[] update = new Node[levelCount];
+        Node p = head;
+        for (int i = levelCount - 1; i >= 0; --i) {
+            while (p.forwards[i] != null && p.forwards[i].data < value) {
+                p = p.forwards[i];
+            }
+            update[i] = p;
+        }
+
+        if (p.forwards[0] != null && p.forwards[0].data == value) {
+            for (int i = levelCount - 1; i >= 0; --i) {
+                if (update[i].forwards[i] != null && update[i].forwards[i].data == value) {
+                    update[i].forwards[i] = update[i].forwards[i].forwards[i];
+                }
+            }
+        }
+    }
+
+    // 随机 level 次，如果是奇数层数 +1，防止伪随机
+    private int randomLevel() {
+        int level = 1;
+        for (int i = 1; i < MAX_LEVEL; ++i) {
+            if (r.nextInt() % 2 == 1) {
+                level++;
+            }
+        }
+
+        return level;
+    }
+
+    public void printAll() {
+        Node p = head;
+        while (p.forwards[0] != null) {
+            System.out.print(p.forwards[0] + " ");
+            p = p.forwards[0];
+        }
+        System.out.println();
+    }
+
+    public class Node {
+        private int data = -1;
+        private Node forwards[] = new Node[MAX_LEVEL];
+        private int maxLevel = 0;
+    }
+
+}

lab-09/src/main/java/cn/iocoder/springboot/labs/lab09/search/SkipList2.java

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+package cn.iocoder.springboot.labs.lab09.search;
+
+import java.util.Random;
+
+/**
+ * 参考 {@link SkipList} ，自己实现一遍跳表
+ *
+ * @author yunai
+ */
+@SuppressWarnings("Duplicates")
+public class SkipList2 {
+
+    private static final int MAX_LEVEL = 16;
+
+    private Random random = new Random();
+
+    /**
+     * 总层级数
+     */
+    private int levelCount = 1;
+
+    private Node head = new Node(null, MAX_LEVEL);
+
+    public Node find(int value) {
+        // 自最上层索引，开始往下查询
+        Node p = head;
+        for (int i = levelCount - 1; i >= 0; i--) {
+            while (p.forwards[i] != null && p.forwards[i].data < value) { // 这里看的会有点绕，第一次 for ，跳转到对应 i 层，后续的，就是第 i 层的不断向下指向
+                p = p.forwards[i];
+            }
+        }
+
+        // 判断是否相等
+        if (p.forwards[0] != null && p.forwards[0].data == value) {
+            return p.forwards[0];
+        }
+        return null;
+    }
+
+    public void insert(int value) {
+        // 创建 Node
+        int maxLevel = randomLevel();
+        Node node = new Node(value, maxLevel);
+
+        // 寻找每一层的指向
+        Node[] update = new Node[maxLevel];
+//        for (int i = 0; i < maxLevel; i++) { // 初始化 node 在一层的指向 。不需要，因为下面的 for 循环，一定会给 update 数组赋值。
+//            update[i] = head;
+//        }
+        Node p = head;
+        for (int i = maxLevel - 1; i >= 0; i--) {
+            while (p.forwards[i] != null && p.forwards[i].data < value) { // 这里看的会有点绕，第一次 for ，跳转到对应 i 层，后续的，就是第 i 层的不断向下指向
+                p = p.forwards[i];
+            }
+            update[i] = p;
+        }
+
+        // 设置指向
+        for (int i = 0; i < maxLevel; i++) {
+            // 设置 node 在第 i 层，指向 update[i].forwards[i]
+            node.forwards[i] = update[i].forwards[i];
+            // 将 update[i].forwards[i] 赋值成 node 。
+            // 这样，就形成了 update[i].forwards[i] = node ，并且 node..forwards[i] = 原 update[i].forwards[i] 。可能有点绕，可以调试下
+            update[i].forwards[i] = node;
+        }
+
+        // 设置新的最高高度
+        if (maxLevel > levelCount) {
+            levelCount = maxLevel;
+        }
+    }
+
+    public void delete(int value) {
+        // 寻找一层对 value 应该 Node 的指向
+        Node[] update = new Node[levelCount];
+        Node p = head;
+        for (int i = levelCount - 1; i >= 0; i--) {
+            while (p.forwards[i] != null && p.forwards[i].data < value) { // 这里看的会有点绕，第一次 for ，跳转到对应 i 层，后续的，就是第 i 层的不断向下指向
+                p = p.forwards[i];
+            }
+            update[i] = p;
+        }
+
+        // 如果找到指定节点
+        if (p.forwards[0] != null && p.forwards[0].data == value) {
+            for (int i = levelCount - 1; i >= 0; i--) {
+                // 指定层，有符合 value 应该 Node 的指向，进行删除
+                if (update[i].forwards[i] != null && update[i].forwards[i].data == value) {
+                    update[i].forwards[i] = update[i].forwards[i].forwards[i];
+                }
+            }
+        }
+    }
+
+    private int randomLevel() {
+        return random.nextInt(MAX_LEVEL - 1) + 1; // 一定建立索引，避免直接添加到第 0 层。不然可能会丢失。
+//         return random.nextInt(MAX_LEVEL); // 因为我们是从 1 层开始计数，可以看 levelCount 参数
+    }
+
+//    // 随机 level 次，如果是奇数层数 +1，防止伪随机
+//    private int randomLevel() {
+//        int level = 1;
+//        for (int i = 1; i < MAX_LEVEL; ++i) {
+//            if (random.nextInt() % 2 == 1) {
+//                level++;
+//            }
+//        }
+//
+//        return level;
+//    }
+
+    public class Node {
+
+        /**
+         * 数值
+         */
+        private Integer data;
+
+        // TODO 芋艿，如果想实现类似 HashMap 的功能，可以把 data 改成 key + value 。
+
+        /**
+         * 最高层级
+         */
+        private int maxLevel;
+        /**
+         * 对指定层级的指向
+         */
+        private Node[] forwards;
+
+        public Node(Integer data, int maxLevel) {
+            this.data = data;
+            this.maxLevel = maxLevel;
+            this.forwards = new Node[maxLevel];
+//            this.forwards = new Node[MAX_LEVEL];
+        }
+
+        @Override
+        public String toString() {
+            return "Node{" +
+                    "data=" + data +
+                    ", maxLevel=" + maxLevel +
+                    '}';
+        }
+    }
+
+}

lab-09/src/main/java/cn/iocoder/springboot/labs/lab09/search/SkipListTest.java

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+package cn.iocoder.springboot.labs.lab09.search;
+
+public class SkipListTest {
+
+    public static void main2(String[] args) {
+        SkipList skipList = new SkipList();
+        skipList.insert(1);
+        skipList.insert(2);
+        skipList.insert(3);
+        skipList.insert(1);
+        skipList.insert(1);
+        skipList.insert(1);
+        skipList.delete(1);
+        skipList.printAll();
+    }
+
+    public static void main(String[] args) {
+        for (int i = 0; i < 10000; i++) {
+        SkipList2 skipList = new SkipList2();
+//            SkipList skipList = new SkipList();
+            // 测试添加
+            skipList.insert(2);
+            skipList.insert(1);
+
+            // 测试查询
+            System.out.println(skipList.find(1) != null);
+            assert skipList.find(1) != null;
+            System.out.println(skipList.find(2) != null);
+            assert skipList.find(2) != null;
+            System.out.println(skipList.find(3) == null); // null
+            assert skipList.find(3) == null;
+
+            // 测试删除
+            skipList.delete(1);
+            System.out.println(skipList.find(1) == null); // null
+            assert skipList.find(1) == null;
+            System.out.println(skipList.find(2) != null);
+            assert skipList.find(2) != null;
+        }
+    }
+
+}