java算法小例子

作为一个程序员，有时候我觉得自己都不适合，因为自己数学不好，算法不好，脑子不够灵活。而对于算法，感觉就像是数学题，总觉得很难。以前上学，在班里总有几个什么都不好，但唯独数学很厉害，真气人！面对难题时，我只能求助，自己想破脑瓜子都想不出什么好的解决办法，都不知道怎么提高这方面的能力。只能慢慢学习了。

自己买了本算法的书，有空的时候就看看，写一写，总是收获的！下面是几个例子：

1.判断闰年

public class LeapYear {
    public static int leapYear(int year) {
        if ((year % 400 == 0) || (year % 100 != 0) && (year % 4 == 0)) {
            return 1; //is leap year
        } else {
            return 0; //not a leap year
        }
    }

    public static void main(String[] args) {
        int year;
        int count = 0;
        System.out.println("2000年到3000年之间所有的闰年如下：");
        for (year = 2000; year <= 3000; year++) {
            if (leapYear(year) == 1) {
                System.out.print(year + " ");
                count++;
                if (count % 16 == 0) {
                    System.out.println();
                }
            }
        }
        System.out.println("一共有" + count + "个闰年！");
    }
}

2.水仙花数

public class NarcissusNum {

    public static void narcissusNum(int n) {
        long i,start,end,temp,num,sum;
        int j;

        start = (long) Math.pow(10, n - 1);
        end = (long) (Math.pow(10, n) - 1);

        for (i = start; i <= end; i++) {
            temp = 0;
            num = i;
            sum = 0;
            for (j = 0; j < n; j++) {
                temp = num % 10;
                sum = (long) (sum + Math.pow(temp, n));
                num = (num - temp) / 10;
            }
            if (sum == i) {
                System.out.printf("%d\n", i);
            }
        }
    }

    public static void main(String[] args) {
        int n;
        n = 3;
        System.out.printf("列举%d位的水仙花数：\n", n);

        narcissusNum(n);
        System.out.println();
        n = 4;
        System.out.printf("列举%d位的水仙花数：\n", n);
        narcissusNum(n);
        System.out.printf("\n");
    }
}

3.判断素数

public class IsPrime {

    public static int isPrime(int a) {
        for (int i = 2; i < a; i++) {
            if (a % i == 0) {
                return 0; //not prime number
            }
        }
        return 1; //is a prime number
    }
    public static void main(String[] args) {
        int i,n,count;
        n = 1000;
        count = 0;
        System.out.println("列举1~1000之间所有的素数");
        for (i = 1; i < 1000; i++) {
            if (isPrime(i) == 1) {
                System.out.printf("%7d", i);
                count++;
                if (count % 10 == 0) {
                    System.out.println();
                }
            }
        }
        System.out.println();
    }
}

4. List去重

public static List removeByFor(List list) {
        for (int i = 0; i < list.size() - 1; i++) {
            for (int j = list.size() - 1; j > i; j--) {
                if (list.get(i).equals(list.get(j))) {
                    list.remove(j);
                }
            }
        }
        return list;
    }

    public static List removeBySet(List list) {
        HashSet h = new HashSet(list);
        list.clear();
        list.addAll(h);
        return list;
    }

    public static List removeDuplicateWithOrder(List list) {
        Set set = new HashSet();
        List newList = new ArrayList();
        for (Iterator iterator = list.iterator(); iterator.hasNext();) {
            Object element = iterator.next();
            if (set.add(element)) {
                newList.add(element);
            }
        }
        list.clear();
        list.addAll(newList);
        return list;
    }

    public static List removeByContain(List list) {
        List tempList = new ArrayList();
        for (int i = 0;i < list.size(); i++) {
            if (!tempList.contains(list.get(i))) {
                tempList.add(list.get(i));
            }
        }
        return tempList;
    }

5.Map与Bean相互转换

public static Object mapToObject(Map<String, Object> map, Class<?> beanClass) throws Exception {
        if (map == null) {
            return null;
        }
        ConvertUtils.register((aClass, o) -> {
            SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
            try {
                return simpleDateFormat.parse(o.toString());
            } catch (Exception e) {
                e.printStackTrace();
            }
            return null;
        }, Date.class);

        Object obj = beanClass.newInstance();
        BeanUtils.populate(obj, map);
        return obj;
    }

    public static Map<String, Object> objectToMap(Object obj) throws Exception {
        return BeanUtils.describe(obj);
    }

    public static Map<String, Object> objectToMapTwo(Object obj) throws Exception {
        if (obj == null) {
            return Maps.newHashMap();
        }
        Map<String, Object> map = Maps.newHashMap();
        Field[] declaredFields = obj.getClass().getDeclaredFields();
        for (Field field : declaredFields) {
            field.setAccessible(true);
            map.put(field.getName(), field.get(obj));
        }
        return map;
    }

    public static Object mapToObjectTwo(Map<String, Object> map, Class<?> beanClass) throws Exception {
        if (map == null) {
            return null;
        }
        Object obj = beanClass.newInstance();
        Field[] fields = obj.getClass().getDeclaredFields();
        for (Field field : fields) {
            int mod = field.getModifiers();
            if (Modifier.isStatic(mod) || Modifier.isFinal(mod)) {
                continue;
            }
            field.setAccessible(true);
            field.set(obj, map.get(field.getName()));
        }
        return obj;
    }

6.排序

    //插入
    public static void insertSort(int[] a) {
        int length = a.length;
        for (int i = 1; i < length; i++) {
            int insertNum = a[i];
            int j = i - 1;
            while (j >= 0 && a[j] > insertNum) {
                a[j + 1] = a[j];
                j--;
            }
            a[j + 1] = insertNum;
        }
    }
    //希尔
    public static void sheelSort(int a[]) {
        int length = a.length;
        for (int gap = length / 2; gap > 0; gap /= 2) {
            for (int i = gap; i < length; i++) {
                int insertNum = a[i];
                int j = i - gap;
                while (j >= 0 && a[j] > insertNum) {
                    a[j + gap] = a[j];
                    j -= gap;
                }
                a[j + gap] = insertNum;
            }
        }
    }

    //冒泡
    public static void bubble(int[] a) {
        int length = a.length;
        int temp;
        for (int i = 0; i < a.length; i++) {
            for (int j = 0; j < a.length - i - 1; j++) {
                if (a[j] > a[j + 1]) {
                    temp = a[j];
                    a[j] = a[j + 1];
                    a[j + 1] = temp;
                }
            }
        }
    }
    //简单选择排序
    public static void selectSort(int[] a) {
        int length = a.length;
        for (int i = 0; i < length; i++) {
            //记录当前最小数和位置
            int key = a[i];
            int position = i;
            for (int j = i + 1; j < length; j++) {
                if (key > a[j]) {
                    key = a[j];
                    position = j;
                }
            }
            //交换
            a[position] = a[i];
            a[i] = key;
        }
    }

    //快速排序
    public static void quickSort(int[] a, int start, int end) {
        if (a == null || start >= end) return;

        int key = a[start];
        int i = start, j = end;

        while (i < j) {
            while (i < j && a[j] >= key) {
                j--;
            }
            if (i < j) {
                a[i++] = a[j];
            }
            while (i < j && a[i] <= key) {
                i++;
            }
            if (i < j) {
                a[j--] = a[i];
            }
        }
        a[i] = key;
        quickSort(a, start, i - 1);
        quickSort(a, i + 1, end);
    }

    //基数排序
    public static void radixSort(int[] a) {
        int queueNum = 10;
        int length = a.length;
        //找到最大数，判断位数
        int max = a[0];
        for (int i = 1; i < length; i++) {
            if (a[i] > max) {
                max = a[i];
            }
        }
        int times = 0;
        while (max > 0) {
            max /= 10;
            times++;
        }
        //初始化10个队列
        ArrayList<ArrayList> queue = new ArrayList<>();
        for (int i = 0; i < queueNum; i++) {
            ArrayList<Integer> queue1 = new ArrayList<>();
            queue.add(queue1);
        }

        for (int i = 0; i < times; i++) {
            //分配数组元素
            for (int j = 0; j < length; j++) {
                //得到位数
                int x = a[j] % (int) Math.pow(10, i + 1) / (int) Math.pow(10, i);
                ArrayList<Integer> queue2 = queue.get(x);
                queue2.add(a[j]);
                queue.set(x, queue2);
            }
            //记录元素数
            int count = 0;
            //收集队列元素
            for (int k = 0; k < queueNum; k++) {
                while (queue.get(k).size() > 0) {
                    ArrayList<Integer> queue3 = queue.get(k);
                    a[count] = queue3.get(0);
                    queue3.remove(0);
                    count++;
                }
            }
        }

    }

    //堆排序
    public static void heapSort(int[] a) {
        int length = a.length;
        for (int i = 0; i < length - 1; i++) {
            adjustHeap(a, length - i - 1);
            swap(a, 0, length - i - 1);
        }
    }

    public static void adjustHeap(int[] a, int lastIndex) {
        for (int i = (lastIndex - 1) / 2; i >= 0 ; i--) {
            int k = i;
            while (2*k + 1 <= lastIndex) {
                int left = 2*k + 1;
                if (left < lastIndex && a[left] < a[left + 1]) {
                    left++;
                }
                if (a[k] >= a[left]) break;
                swap(a, k, left);
                k = left;
            }

        }
        System.out.println("调整后" + Arrays.toString(a));
    }

    //归并排序
    public static void mergeSortTwo(int[] a, int start, int end) {
        if (start < end) {
            int mid = (start + end) / 2;
            mergeSortTwo(a, start, mid);
            mergeSortTwo(a, mid + 1, end);
            mergeTwo(a, start, mid, end);
        }
    }
    public static void mergeTwo(int[] a, int start, int mid, int end) {
        int[] temp = new int[a.length];
        int p1 = start, p2 = mid + 1, k = start;
        while (p1 <= mid && p2 <= end) {
            if (a[p1] <= a[p2]) {
                temp[k++] = a[p1++];
            } else {
                temp[k++] = a[p2++];
            }
        }
        while (p1<=mid) {
            temp[k++] = a[p1++];
        }
        while (p2<=end) {
            temp[k++] = a[p2++];
        }
        for (int i = start; i <= end; i++) {
            a[i] = temp[i];
        }
    }