package array;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Stack;
public class ArrayUtil {
public static void main(String[] args) {
int[] origin = new int[]{1, 2, 3, 4};
System.out.println(Arrays.toString(getPrimes(23)));
}
/**
* 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]
* @param origin
* @return
*/
/**
* 1. 通过双指针实现反转
* @param origin
* @return
*/
public static int[] reverseArray_1(int[] origin){
int i = 0;
int j = origin.length - 1;
for (;i < j;) {
int swap = origin[i];
origin[i] = origin[j];
origin[j] = swap;
i++;
j--;
}
return origin;
}
/**
* 2. 通过栈后进先出的特点实现反转
* @param origin
* @return
*/
public static int[] reverseArray_2(int[] origin) {
Stack<Integer> stack = new Stack<>();
for (int num : origin) {
stack.push(num);
}
for (int i = 0; i < origin.length; i++) {
origin[i] = stack.pop();
}
return origin;
}
/**
* 3. 通过反序遍历实现反转
* @param origin
* @return
*/
public static int[] reverseArray_3(int[] origin) {
int[] result = new int[origin.length];
for (int i = origin.length - 1; i >= 0; i--) {
result[origin.length - i - 1] = origin[i];
}
return result;
}
/**
* 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}
* @param oldArray
* @return
*/
public static int[] removeZero_1(int[] oldArray){
List<Integer> list = new ArrayList<>();
for (int num : oldArray) {
if (num != 0) {
list.add(num);
}
}
int[] result = new int[list.size()];
for (int i = 0; i < list.size(); i++) {
result[i] = list.get(i);
}
return result;
}
/**
* 通过双指针实现
* @param oldArray
* @return
*/
public static int[] removeZero_2(int[] oldArray) {
int j = 0;
for (int i = 0; i < oldArray.length; i++) {
if (oldArray[i] != 0) {
oldArray[j++] = oldArray[i];
}
}
return Arrays.copyOf(oldArray, j);
}
/**
* 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
* @param array1
* @param array2
* @return
*/
public int[] merge(int[] array1, int[] array2){
return null;
}
/**
* 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]
* @param oldArray
* @param size
* @return
*/
public static int[] grow(int [] oldArray, int size){
return Arrays.copyOf(oldArray, oldArray.length + size);
}
/**
* 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 []
* @param max
* @return
*/
public static int[] fibonacci(int max){
List<Integer> list = new ArrayList<>();
int current = 0;
for (int i = 0; i < max; i++) {
if (i <= 1) {
current = 1;
} else {
current = list.get(i - 1) + list.get(i - 2);
}
if (current >= max) break;
list.add(current);
}
int[] result = new int[list.size()];
for (int i = 0; i < list.size(); i++) {
result[i] = list.get(i);
}
return result;
}
/**
* 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]
*
* 写了一篇文章:http://bosschow.github.io/2017/03/05/get-prime-number-1/
* @param max
* @return
*/
public static int[] getPrimes(int max){
List<Integer> list = new ArrayList<>();
for (int i = 2; i < max; i++) {
boolean isPrime = true;
for (Integer primeNum : list) {
if (primeNum <= Math.sqrt(i)) {
if (i % primeNum == 0) {
isPrime = false;
break;
}
} else {
break;
}
}
if (isPrime) {
list.add(i);
}
}
int[] result = new int[list.size()];
for (int i = 0; i < list.size(); i++) {
result[i] = list.get(i);
}
return result;
}
/**
* 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
* @param max
* @return
*/
public int[] getPerfectNumbers(int max){
return null;
}
/**
* 用seperator 把数组 array给连接起来
* 例如array= [3,8,9], seperator = "-"
* 则返回值为"3-8-9"
* @param array
* @param s
* @return
*/
public static String join(int[] array, String seperator){
StringBuilder stringBuilder = new StringBuilder();
for (int i = 0; i < array.length; i++) {
if (i == array.length - 1) {
stringBuilder.append(array[i]);
} else {
stringBuilder.append(array[i] + seperator);
}
}
return stringBuilder.toString();
}
}