上一篇 数据结构与算法学习-数组 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> 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<Integer> 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<Integer> 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<Integer> list = new RArrayList<>();
for (int i = 2; i < max; i++) {
if (isPrime(i)) {
list.add(i);
}
}
return listToArr(list);
}
private static int[] listToArr(RList<Integer> 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<Integer> 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);
}
}
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