本文共 8814 字，大约阅读时间需要 29 分钟。
上一篇 简单实现了一下 ArrayList，对 List 的增删改查有了一定的认识，这一部分，来练习一下一些简单小算法，可以使用我们自己实现的 ArrayList，顺便检查下哪里写的不好。
下面就直接给出算法，具体的逻辑不讲解，都是相对比较基础的算法题，花一点时间相信大家能够看得懂！
public class RArrayListUtil { /** * 给定一个整形数组a , 对该数组的值进行置换 * 例如： a = [7, 9 , 30, 3] , 置换后为 [3, 30, 9,7] * 如果 a = [7, 9, 30, 3, 4] , 置换后为 [4,3, 30 , 9,7] */ public static void reverseArray(int[] origin) { if (origin == null || origin.length < 1) { return; } for (int i = 0; i < origin.length / 2; i++) { swap(origin, i, origin.length - i - 1); } } private static void swap(int[] list, int indexA, int indexB) { if (list != null && indexA < list.length && indexB < list.length) { int temp = list[indexA]; list[indexA] = list[indexB]; list[indexB] = temp; } } /** * 现在有如下的一个数组： int oldArr[]={1,3,4,5,0,0,6,6,0,5,4,7,6,7,0,5} * 要求将以上数组中值为0的项去掉，将不为0的值存入一个新的数组，生成的新数组为： * {1,3,4,5,6,6,5,4,7,6,7,5} */ public static int[] removeZero(int[] oldArray) { if (oldArray == null || oldArray.length < 1) return null; int count = 0; for (int i : oldArray) { if (i != 0) count++; } int[] newArray = new int[count]; int i = 0; for (int val : oldArray) { if (val != 0) { newArray[i++] = val; } } return newArray; } /** * 给定两个已经排序好的整形数组， a1和a2 , 创建一个新的数组a3, 使得a3 包含a1和a2 的所有元素， 并且仍然是有序的 * 例如 a1 = [3, 5, 7,8] a2 = [4, 5, 6,7] 则 a3 为[3,4,5,6,7,8] , 注意： 已经消除了重复 */ public static int[] merge(int[] array1, int[] array2) { if (array1 == null || array1.length < 1) { return array2; } if (array2 == null || array2.length < 1) { return array1; } int[] mergeArray = new int[array1.length + array2.length]; int index1 = 0; int index2 = 0; int mergeLength = 0; for (int i = 0; i < mergeArray.length; i++) { if (index1 == array1.length) { mergeLength += array2.length - index2; System.arraycopy(array2, index2, mergeArray, i, array2.length - index2); break; } else if (index2 == array2.length) { mergeLength += array1.length - index1; System.arraycopy(array1, index1, mergeArray, i, array1.length - index1); break; } else { int num1 = array1[index1]; int num2 = array2[index2]; mergeLength++; if (num1 > num2) { mergeArray[i] = num2; index2++; } else if (num1 < num2) { mergeArray[i] = num1; index1++; } else { mergeArray[i] = num1; index1++; index2++; } } } // 清除0部分 int[] result = new int[mergeLength]; System.arraycopy(mergeArray, 0, result, 0, result.length); return result; } /** * 把一个已经存满数据的数组 oldArray的容量进行扩展， 扩展后的新数据大小为oldArray.length + size * 注意，老数组的元素在新数组中需要保持 * 例如 oldArray = [2,3,6] , size = 3,则返回的新数组为 * [2,3,6,0,0,0] */ @SuppressWarnings("unchecked") public static T[] grow(T[] oldArray, int size) { if (oldArray == null || oldArray.length < 1) { return null; } T[] newArray = (T[]) new Object[oldArray.length + size]; int i = 0; for (T item : oldArray) { newArray[i++] = item; } return newArray; } /** * 斐波那契数列为：1，1，2，3，5，8，13，21...... ，给定一个最大值， 返回小于该值的数列 * 例如， max = 15 , 则返回的数组应该为 [1，1，2，3，5，8，13] * max = 1, 则返回空数组 [] */ public static int[] fibonacci(int max) { if (max < 2) { return null; } RList list = new RArrayList<>(max); // 第一个数 list.add(1); getFibonacci(1, 1, max, list); return listToArr(list); } private static void getFibonacci(int a, int b, int max, RList list) { if (b > max) { return; } list.add(b); getFibonacci(b, a + b, max, list); } /** * 返回小于给定最大值max的所有素数数组 * 例如max = 23, 返回的数组为[2,3,5,7,11,13,17,19] */ public static int[] getPrimes(int max) { if (max < 2) { return null; } RList list = new RArrayList<>(); for (int i = 2; i < max; i++) { if (isPrime(i)) { list.add(i); } } return listToArr(list); } private static int[] listToArr(RList list) { int[] arr = new int[list.size()]; for (int i = 0; i < list.size(); i++) { arr[i] = list.get(i); } return arr; } private static boolean isPrime(int num) { if (num < 4) { return true; } boolean isPrime = true; for (int i = 2; i <= num / 2; i++) { if (num % i == 0) { isPrime = false; break; } } return isPrime; } /** * 所谓“完数”， 是指这个数恰好等于它的因子之和，例如6=1+2+3 * 给定一个最大值max， 返回一个数组， 数组中是小于max 的所有完数 */ public static int[] getPerfectNumbers(int max) { if (max < 2) { return null; } RList list = new RArrayList<>(); for (int i = 2; i <= max; i++) { if (isPerfectNumber(i)) { list.add(i); } } return listToArr(list); } private static boolean isPerfectNumber(int num) { if (num < 2) { return false; } int sum = 1; for (int i = 2; i <= num / 2; i++) { if (num % i == 0) { sum += i; } } if (num == sum) { return true; } return false; } /** * 用separator 把数组 array给连接起来 * 例如array= [3,8,9], separator = "-" * 则返回值为"3-8-9" */ public static String join(int[] array, String separator) { if (array == null || array.length < 1) { return null; } StringBuilder builder = new StringBuilder(); for (int i = 0; i < array.length; i++) { if (i == array.length - 1) { builder.append(array[i]); } else { builder.append(array[i]).append(separator); } } return builder.toString(); }} 测试代码
public class RArrayListUtilTest { int[] arr1; int[] arr2; @Before public void initArr() { arr1 = new int[]{ 7, 9, 30, 3}; arr2 = new int[]{ 7, 9, 30, 3, 4}; } @Test public void reverseArray() { RArrayListUtil.reverseArray(arr1); Assert.assertArrayEquals(new int[]{ 3, 30, 9, 7}, arr1); RArrayListUtil.reverseArray(arr2); Assert.assertArrayEquals(new int[]{ 4, 3, 30, 9, 7}, arr2); } @Test public void removeZero() { int[] arr = new int[]{ 0, 7, 0, 9, 0, 30, 3, 0}; int[] result = RArrayListUtil.removeZero(arr); Assert.assertEquals(4, result.length); Assert.assertArrayEquals(new int[]{ 7, 9, 30, 3}, result); } @Test public void merge() { int[] a1 = { 3, 5, 7, 8}; int[] a2 = { 4, 5, 6, 7}; // a3 为[3,4,5,6,7,8] int[] result = RArrayListUtil.merge(a1, a2); Assert.assertEquals(6, result.length); Assert.assertArrayEquals(new int[]{ 3, 4, 5, 6, 7, 8}, result); } @Test public void grow() { String[] strings = new String[]{ "Hello", "World", "!"}; Object[] grow = RArrayListUtil.grow(strings, 5); Assert.assertEquals(8, grow.length); Assert.assertEquals("Hello", grow[0]); Assert.assertEquals("World", grow[1]); Assert.assertEquals("!", grow[2]); for (int i = 3; i < grow.length; i++) { Assert.assertNull(grow[i]); } } @Test public void fibonacci() { int[] fibonacci = RArrayListUtil.fibonacci(15); Assert.assertEquals(7, fibonacci.length); Assert.assertArrayEquals(new int[]{ 1, 1, 2, 3, 5, 8, 13}, fibonacci); } @Test public void getPrimes() { int[] primes = RArrayListUtil.getPrimes(23); Assert.assertEquals(8, primes.length); Assert.assertArrayEquals(new int[]{ 2, 3, 5, 7, 11, 13, 17, 19}, primes); } @Test public void getPerfectNumbers() { int[] perfectNumbers = RArrayListUtil.getPerfectNumbers(10000); Assert.assertEquals(4, perfectNumbers.length); Assert.assertArrayEquals(new int[]{ 6, 28, 496, 8128}, perfectNumbers); } @Test public void join() { String join = RArrayListUtil.join(arr1, "-"); Assert.assertEquals("7-9-30-3", join); join = RArrayListUtil.join(arr2, "-"); Assert.assertEquals("7-9-30-3-4", join); }} 转载地址：http://jidws.baihongyu.com/