java SkipList

package ds;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class SkipList<T> {
    
    
    private static class SkipNode<T>{
    
    
        int key;
        T value;
        SkipNode<T> next;
        SkipNode<T> down;

        public SkipNode() {
    
    
            this(Integer.MIN_VALUE,null);
        }

        public SkipNode(int key, T value) {
    
    
            this.key = key;
            this.value = value;
        }
    }

    private static final int MAX_LEVEL = 1 << 6;
    private int level = 0;
    private SkipNode<T> top = null;
    private Random random = new Random();

    public SkipList() {
    
    
        this(4);
    }

    public SkipList(int level) {
    
    
        this.level = level;
        SkipNode<T> tail = null;
        while(level-- != 0){
    
    
            SkipNode<T> tempTop = new SkipNode<>();
            SkipNode<T> tempTail = new SkipNode<>(Integer.MAX_VALUE,null);

            tempTop.next = tempTail;
            tempTop.down = top;
            tempTail.down = tail;

            top = tempTop;
            tail = tempTop;
        }
    }

    private int getRqandomLevel(){
    
    
        int lev = 1;
        while(random.nextInt() % 2 == 0){
    
    
            ++lev;
        }
        return lev > MAX_LEVEL ? MAX_LEVEL : lev;
    }


    public T get(int key){
    
    
        SkipNode<T> temp = top;
        while(temp != null){
    
    
            while(key < temp.next.key){
    
    
                temp = temp.next;
            }
            if(key == temp.next.key){
    
    
                return temp.next.value;
            }
            temp = temp.down;
        }
        return null;
    }


    @Override
    public String toString(){
    
    
        StringBuilder sb = new StringBuilder();
        SkipNode<T> temp = top;
        while(temp.down != null){
    
    
            temp = temp.down;
        }
        while(temp.next.key != Integer.MIN_VALUE){
    
    
            sb.append(" (" + temp.next.key + "->" + temp.next.value + ") ");
            temp = temp.next;
        }
        return sb.toString();
    }

    public void printLevel(){
    
    
        SkipNode<T> temp = top;
        int lev = level;
        while(temp != null){
    
    
            SkipNode<T> tempStart = temp;
            StringBuilder sb = new StringBuilder();
            while(tempStart.next.key != Integer.MAX_VALUE){
    
    
                sb.append(" (" + tempStart.next.key + "->" + tempStart.next.value + ") ");
                tempStart = tempStart.next;
            }
            System.out.print("第" + lev-- + "层：");
            System.out.println(sb.toString());
            temp = temp.down;
        }
    }


    /**
     * 返回插入key的前去节点链表
     */
    private List<SkipNode<T>> getPreNodeList(int key){
    
    
        boolean isExisted = false;
        SkipNode<T> temp = top;
        List<SkipNode<T>> ret = new ArrayList<>();
        while(temp != null){
    
    
            while(key < temp.next.key){
    
    
                temp = temp.next;
            }
            if(isExisted == false && temp.next.key == key){
    
    
                isExisted = true;
                ret.clear();
            }
            ret.add(temp);
            temp = temp.down;
        }
        if(isExisted == false){
    
    
            ret.add(null);
        }
        return ret;
    }

    public boolean delete(int key){
    
    
        List<SkipNode<T>> preNodeList = getPreNodeList(key);
        if(preNodeList.get(preNodeList.size() - 1) == null){
    
    
            return false;
        }
        for(SkipNode<T> node : preNodeList){
    
    
            SkipNode<T> del = node.next;
            node.next = del.next;
            del.down = null;
        }
        return true;
    }


    public void put(int key,T value){
    
    
        List<SkipNode<T>> preNodeList = getPreNodeList(key);
        if(preNodeList.get(preNodeList.size() - 1) != null){
    
    
            preNodeList.stream().forEach(node ->node.next.value = value);
            return ;
        }
        int lev = getRqandomLevel();
        int len = preNodeList.size() - 1;
        SkipNode<T> down = null;
        while(lev-- != 0 && len-- != 0){
    
    
            SkipNode<T> node = new SkipNode<>(key, value);
            SkipNode<T> node1 = preNodeList.get(len);
            node.next = node1.next;
            node1.next = node;
            node.down = down;
            down = node;
        }
        if(lev == -1){
    
    
            return;
        }
        ++lev;
        SkipNode<T> tempTop = null;
        SkipNode<T> tempTail = null;
        SkipNode<T> tail = top;
        while(tail.next != null){
    
    
            tail = tail.next;
        }
        while(lev-- != 0){
    
    
            tempTop = new SkipNode<>(Integer.MIN_VALUE, null);
            SkipNode<T> temp = new SkipNode<>(key, value);
            tempTail = new SkipNode<>(Integer.MAX_VALUE,null);

            tempTop.next = temp;
            temp.next = tempTail;

            tempTop.down = top;
            temp.down = down;
            tempTail.down = tail;

            top = tempTop;
            down = temp;
            tail = tempTail;
        }
    }
}