Java数组相关算法一

一、数组反转　

　　1、方法一：创建新数组

 int[] arr = {6,29,0,4,3};
 int[] arr2 = new int[arr.length];
 for (int i = 0; i < arr.length; i++) {
     arr2[i]=arr[arr.length-1-i];
 }
 System.out.println(Arrays.toString(arr2));

　　2、方法二：交换

 int[] arr = {6,29,0,4,3};
 int mid = arr.length/2;
 for (int i = 0,j=arr.length-1; i < mid; i++,j--) {
     int a = arr[i];
     arr[i] = arr[j];
     arr[j] = a;
 }
 System.out.println(Arrays.toString(arr));

二、查找数组中指定元素的索引位置

　　1、方法一：遍历

　　

 public class FindArraryValue {
     public static void main(String[] args) {
         int[] arr = { 6, 29, 1, 4, 3,100,80,1000,600 };
         System.out.println(find1(arr, 6));//
     }

     public static int findValue(int[] a, int v) {
         for (int i = 0; i < a.length; i++) {
             if (a[i] == v) {
                 return i;
             }
         }
         return -1;
     }
  }

　　2、方法二：二分查找法（必须先排序）

 public class FindArraryValue {
     public static void main(String[] args) {
         int[] arr = { 6, 29, 1, 4, 3,100,80,1000,600 };
         Arrays.sort(arr);
         System.out.println(Arrays.toString(arr));
         //[1, 3, 4, 6, 29, 80, 100, 600, 1000]
         System.out.println(binaryResearch(arr, 0, arr.length, 6));
         //[1, 3, 4, 6, 29, 80, 100, 600, 1000]
         System.out.println(Arrays.binarySearch(arr, 6));
     }

     public static int binaryResearch(int[] a, int fromIndex, int toIndex, int v) {
         if (a == null) {
             throw new RuntimeException("数组不能为空");
         }
         if (fromIndex < 0) {
             throw new RuntimeException("开始索引不能为负数");
         }
         if (fromIndex > toIndex) {
             throw new RuntimeException("开始索引不能小于结束索引");
         }
         if (fromIndex > a.length) {
             throw new RuntimeException("结束索引不能大于数组的长度");
         }
         int low = fromIndex;
         int high = toIndex - 1;
         while (low <= high) {
             //无符号右移一位，即除以2
             int mid = (high + low) >>> 1;
             if (a[mid] < v) {
                 low = mid + 1;
             } else if (a[mid] > v) {
                 high = mid - 1;
             } else {
                 return mid;
             }
         }
　　　　　　　//未找到
         return -1;
     }
 }

三、杨辉三角

　　1、方法一：直接创建二维数组

 public class Yang {
     public static void main(String[] args) {
         int yang[][] = new int[10][10];

         yang[0][0]=1;
         yang[1][0]=1;
         yang[1][1]=1;
         for (int i = 2; i <10 ; i++) {
             yang[i][0]=1;
             yang[i][i]=1;
             for (int j = 1; j < i; j++) {
                 yang[i][j]=yang[i-1][j-1]+yang[i-1][j];
             }
         }

         for (int i = 0; i <10 ; i++) {
             for (int j = 0; j <= i; j++) {
                 System.out.print(yang[i][j]+"  ");
             }
             System.out.println();
         }
     }
 }    

　　2、方法二：在二层循环中创建一位数组（推荐）

 public class Yang {
     public static void main(String[] args) {
         int yang[][] = new int[10][];
         for (int i = 0; i < yang.length; i++) {
             yang[i]=new int[i+1];
             yang[i][0]=1;
             yang[i][i]=1;
             //for (int j = 1; j < yang[i].length-1; j++) {
             for (int j = 1; j < i; j++) {
                 yang[i][j]=yang[i-1][j-1]+yang[i-1][j];
             }
         }

         for (int i = 0; i <10 ; i++) {
             for (int j = 0; j <= i; j++) {
                 System.out.print(yang[i][j]+"  ");
             }
             System.out.println();
         }
     }
 }

四、九九乘法表　

 public class Jiujiu {
     public static void main(String[] args) {
         for (int i = 1; i < 10; i++) {
             for (int j = 1; j <= i; j++) {
                 System.out.printf("%d * %d = %d   ",j,i,i*j);
             }
             System.out.println();
         }
     }
 }