数组
注意：一旦初始化完成，数组的长度就确定了且！不可更改
创建时，在内存中开辟一整块“连续的空间”，
二维数组
int[][] arr1 = new int[][]{{123},{321,12}};
int[][] arr3 = new int[2][]; // 因为内层未声明，外层元素，默认存储null
// int 外层 内层 变量名 = new int 外层 内层；
/**
声明两个一维数组x,y；
*/
int[] x,y;
// =====
int[] x,y[];
//y是二维，
// x是一维；
常见算法
1、数值型数据的特征统计；（均值，最大、小值；总和）
public static void main(String[] args) {
// 一、求特征值；/*一个数组，10个元素，随机整数，求出最大、小、总和；*//*1、动态初始化*/int[] arr = new int[10];/*2、循环赋值*/for (int i = 0; i < arr.length; i++) {
// arr[i] = (int)(Math.random() * (99 - 10 + 1) + 10); [10-99]arr[i] = (int)(Math.random() * 90 + 10);
// System.out.println(arr[i]);}/*3、求最大值*/int max = arr[0];for (int j = 1; j < arr.length; j++) {if(arr[j] > max) {max = arr[j];}}System.out.println("最大值为：" + max);/*4、求最小值*/
// 略；/*5、求总和*/int total = 0;for (int i = 0; i < arr.length; i++) {total += arr[i];}System.out.println("总和为" + total);/*6、平均值*/int avg = total / arr.length;System.out.println(avg);}
public static void main(String[] args) {
// 特征值；/*评委打分：掐头去尾，求平均值；*/int[] scoreArr = new int[] {5,4,8,9,0,1,2,7,3};int max = scoreArr[0], total = 0, min = scoreArr[0];int avg = 0;for (int i = 0; i < scoreArr.length; i++) {total += scoreArr[i];if(max < scoreArr[i]) {max = scoreArr[i];}if(min > scoreArr[i]) {min = scoreArr[i];}}avg = (total - max -min ) / (scoreArr.length - 2);System.out.println(avg);}
2、数组元素的赋值；
public static void main(String[] args) {
// 数组元素的赋值；/*打印一个10行的， 杨辉三角*//*11 11 2 11 3 3 11 4 6 4 11 5 10 10 5 11 6 15 20 15 6 11 7 21 35 35 21 7 11 8 28 56 70 56 28 8 11 9 36 84 126 126 84 36 9 1 */int[][] arr = new int[10][];for (int i = 0; i < arr.length; i++) {/*定义长度*/arr[i] = new int[i+1];/*首尾赋值为1*/arr[i][0] = 1;arr[i][arr[i].length - 1] = 1; //看图形相当于arr[i][i];
// 条件不满足的不会进来；for (int j = 1; j < arr[i].length - 1; j++) {arr[i][j] = arr[i-1][j] + arr[i-1][j-1];}}/*for (int i = 2; i < arr.length; i++) {for (int j = 1; j < arr[i].length - 1; j++) {arr[i][j] = arr[i-1][j] + arr[i-1][j-1];}}*/for (int i = 0; i < arr.length; i++) {for (int j = 0; j < arr[i].length; j++) {System.out.print(arr[i][j] + "\t");}System.out.println();}// System.out.println();}
3、数组的复制和反转；
引用类型的问题，同JS相同；修改同一个地址后，多个引用同一地址会变动；
一旦初始化完成，数组的长度就确定了且！不可更改
public static void main(String[] args) {
// 数组元素的复制；int[] arr1 = new int[]{1,2,3,4,5,6};int[] arr2 = new int[arr1.length];for (int i = 0; i < arr2.length; i++) {arr2[i] = arr1[i];}System.out.println("==================");/*数组反转*/int[] posArr = new int[] {1,2,3,4,5,6};int[] nagArr = new int[posArr.length];for (int i = 0; i < posArr.length; i++) {nagArr[posArr.length - 1 - i] = posArr[i];}for (int i = 0; i < nagArr.length; i++) {System.out.print(nagArr[i]);}System.out.println("=================");/*数组反转*/
// arr[i] = arr[arr.length - 1 - i]for (int i = 0; i < posArr.length / 2; i++) {int temp = posArr[i];posArr[i] = posArr[posArr.length - 1 - i];posArr[posArr.length - 1 - i] = temp;}for (int i = 0; i < nagArr.length; i++) {System.out.print(posArr[i]);}System.out.println("=================");/*数组反转*/for (int i = 0,j=posArr.length - 1; i < j ; i++, j--) {int mid = posArr[i];posArr[i] = posArr[j];posArr[j] = mid;}for (int i = 0; i < nagArr.length; i++) {System.out.print(posArr[i]);}}
4、扩容和缩容；（扩展长度）
public static void main(String[] args) {/*扩容并添加值10,20,30*/int[] arr = new int[] {1,2,3,4,5};
// int[] newArr = new int[arr.length * 2];int[] newArr = new int[arr.length << 1];for (int i = 0; i < arr.length; i++) {newArr[i] = arr[i];}newArr[arr.length] = 10;newArr[arr.length+1] = 20;newArr[arr.length+2] = 30;arr = newArr;/*GC判断newArr无指针后会进行回收，*/System.out.println("===================");/*缩容，删除第n项*/int index = 4;
// 方式1、int[] oArr = new int[] {1,2,3,4,5,6,7};for (int i = index; i < oArr.length - 1; i++) {oArr[i] = oArr[i+1];}oArr[oArr.length-1] = 0;for (int i = 0; i < oArr.length; i++) {System.out.print(oArr[i] + "\t");}System.out.println("===================");
// 方式2、int[] ooArr = new int[] {1,2,3,4,5,6,7};int[] nArr = new int[ooArr.length - 1];for (int i = 0; i < index; i++) {nArr[i] = ooArr[i];}for (int i = index; i < ooArr.length - 1; i++) {nArr[i]= ooArr[i + 1];}ooArr = nArr;for (int i = 0; i < ooArr.length; i++) {System.out.print(ooArr[i] + "\t");}}
5.元素的查找（线性、二分）
public static void main(String[] args) {/*线性查找*/int[] arr = new int[] {1,2,3,4,5,6,7,8,9,0};int target = 5,index = -1;for (int i = 0; i < arr.length; i++) {if(arr[i] == target) {index = i;break;}}System.out.println("index: " + index);System.out.println("=================");/*二分查找*/
// 前提是一个有序数组；int[] arr2 = new int[] {1,2,3,4,5,6,7,8,34,44,45,56,67,78,89,90};
// int target2 = 8; //查找目标；int target2 = 99; //查找目标；int head = 0, end = arr2.length - 1; //首尾索引；int index2 = -1;while (head <= end) {/*计算中间值*/int mid = (head + end) / 2;if(target2 == arr2[mid]) {index2 = mid;break;} else if(target2 > arr2[mid]) {/*查找右边*/head = mid + 1;} else {/*查找左边*/end = mid - 1;}}System.out.println("index2: " + index2);}
6.数组排序
public static void main(String[] args) {/*排序*//**1、冒泡排序：* 相邻的比较，大的就往后移动；* N轮；* （每次冒个最大的泡泡）*/int[] arr = new int[] {1,2,33,4,5,6,7,8,9,0};for (int i = 0; i < arr.length - 1; i++) {for (int j = 0; j < arr.length - 1 - i; j++) {if(arr[j] > arr[j+1]) {int mid = arr[j];arr[j] = arr[j+1];arr[j+1] = mid;}}}System.out.print("冒泡排序：");for (int i = 0; i < arr.length ; i++) {System.out.print(arr[i] +"\t");}System.out.println();System.out.println("===================");/*快速排序*//****/System.out.print("快速排序：");}
关于部分术语
1、时间复杂度：分析关键字的比较次数和记录的移动次数
：O(1)<log2n < n N nlog2n < n^2 < n^3 <2^n < n! < n^n;2、空间复杂度：分析排序算法中需要多少辅助内存；
很多时候都是用 “”空间换时间。相对来说时间比较重要；3、稳定性：排序后相等的值前后顺序是否可能会变化；
