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));//1
    }

    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();
        }
    }
}