1、單例模式，手寫雙重檢驗單例模式 懶漢式，DCL（8）

餓漢式：
public class Singleton{
    private static Singleton singleton = new Singleton();
    private Singleton(){}
    public static Singleton getSingleton(){
        return singleton;
    }
}

懶漢式(執行緒不安全)：
public class Singleton{
    private static Singleton singleton = null
 
;
    private Singleton(){}
    public static Singleton getSingleton(){
        if(singleton == null){
            singleton = new Singleton();
        }
        return singleton;
    }
}
雙重校驗單例模式(DCL)：
public class Singleton{
    private volatile static Singleton singleton;
    private Singleton(){}
    
 
public static Singleton getSingleton(){
        if(singleton == null){
            //類物件加鎖
            synchronized (Singleton.class) {
                if(singleton == null){
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
}

2、rand4生成rand6

• 給定一個 0-4隨機數生成器 如何生成0-6隨機數

public class Frequency {
    public static int rand7(){
        while(true){
            int num=5*rand5()+rand5();//0-24
            if(num<21)
                return num % 7;
        }
    }
}

//變形：如果用0-6隨機生成器生成0-9隨機數
public class Frequency {
    public static int rand10(){
        while(true){
            int num=7*rand7()+rand7();
            if(num<41)
            //排除41-48，因為他們不能生成9，會造成各個數字出現的概率不同
                return num % 10;
        }
    }
}

3、最長不含重複字元的子字串（劍指offer48題）

雙指標+雜湊表 時間複雜度O(N) 空間複雜度O(1):字元的 ASCII 碼範圍為 0 ~ 127 ，
雜湊表 dicdic 最多使用 O(128) = O(1)大小的額外空間。
class Solution {
    public int lengthOfLongestSubstring(String s) {
        Map<Character, Integer> dic = new HashMap<>();
        int i = -1, res = 0;
        for(int j = 0; j < s.length(); j++) {
            if(dic.containsKey(s.charAt(j)))
                i = Math.max(i, dic.get(s.charAt(j))); // 更新左指標 i
            dic.put(s.charAt(j), j); // 雜湊表記錄
            res = Math.max(res, j - i); // 更新結果
        }
        return res;
    }
}

4、三個執行緒迴圈列印ABC

//使用Lock
public class ABCPrint{
    private static int state = 0;
    public static void main(String[] args) {
        final Lock lock = new ReentrantLock();

        Thread A = new Thread(new Runnable() { <a href="/profile/992988" data-card-uid="992988" class="js-nc-card" target="_blank">@Override public void run() {
                while (state <= 30) {
                    lock.lock();

                    try {
                        if (state % 3 == 0) {
                            System.out.print("A");
                            state++;
                        }
                    } finally {
                        lock.unlock();

                    }
                }
            }
        });

        Thread B = new Thread(new Runnable() { </a><a href="/profile/992988" data-card-uid="992988" class="js-nc-card" target="_blank">@Override public void run() {
                while (state <= 30) {
                    lock.lock();

                    try {
                        if (state % 3 == 1) {
                            System.out.print("B");
                            state++;
                        }
                    } finally {
                        lock.unlock();

                    }
                }
            }
        });

        Thread C = new Thread(new Runnable() { </a><a href="/profile/992988" data-card-uid="992988" class="js-nc-card" target="_blank">@Override public void run() {
                while (state <= 30) {
                    lock.lock();
                    try {
                        if (state % 3 == 2) {
                            System.out.println("C");
                            state++;
                        }
                    } finally {
                        lock.unlock();

                    }
                }
            }
        });
        A.start();
        B.start();
        C.start();
    }
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