java实现二分查找

思路:

　　1.先使用经典算法之快速排序.

　　 2.使用二分查找查找目标数据

代码1：（Quick_Sort.java）

package com.cn.algorithm_arithmetic算法;
/**
 * 本程序记录了经典排序算法之快排
 * 时间复杂度:一般O(nlogn),最差O(n^2)
 * @author Administrator
 *
 */

public class Quick_Sort {
    public static int partition(int a[],int low,int hight){
        int temp = a[low];
        while (low<hight){
            while (low<hight && a[hight]>=temp){
                hight--;
            }
            a[low] = a[hight];
            while (low<hight && a[low]<=temp){
                low++;
            }
            a[hight] = a[low];
            a[low] = temp;
        }
        return low;
    }
    public static void quick_sort(int a[],int low,int hight){
        if (low >= hight){
            return;
        }
        int mid = partition(a, low, hight);
        quick_sort(a,low,mid-1);
        quick_sort(a, mid+1, hight);
    }
    public static int[] get_arr(int size){
        int a[] = new int[size];
        for (int i = 0; i < a.length; i++) {
            a[i] = (int)(Math.random()*1000);
        }
        return a;
    }
    public static void main(String[] args) {
        System.out.println("---------快速排序----------");
        int b[] = new int[10];
        System.out.print("排序前:");
        for (int i = 0; i < b.length; i++) {
            b[i] = (int)(Math.random()*1000);
            System.out.print(b[i]+"  ");
        }
//        long start_time = System.currentTimeMillis();
        System.out.print("
排序后:");
        quick_sort(b, 0, 9);
//        long end_time = System.currentTimeMillis();
        for (int i = 0; i < b.length; i++) {
            System.out.print(b[i]+"  ");
        }
//        System.out.println("
排序总耗时:"+(end_time - start_time));
    }
}

代码2：（Two_Devide_Find.java）

package com.cn.algorithm_arithmetic算法;
/**
 * 二分查找来查找指定的数值
 * @author Administrator
 *
 */
public class Two_Devide_Find {
public static String find_target(int a[],int key){
    int start = 0;
    int end = a.length - 1;
    while (start <= end){
        int mid = (start + end)/2;
        if (a[mid] == key){
            return mid+"";
        }else if (a[mid] > key){
            end = mid - 1;
        }else if (a[mid] < key){
            start = mid + 1;
        }
    }
    return null;
}
public static void main(String[] args) {
    int key = 188;
    int a[] = Quick_Sort.get_arr(1000);
    Quick_Sort.quick_sort(a, 0, 999);
    System.out.print("原始数据:");
    for (int i = 0; i < a.length; i++) {
        if (i%10 == 0){
            System.out.println("
");
        }
        System.out.print(a[i]+"  ");
    }
    String index = find_target(a, key);
    System.out.println("
结果位:"+ (index==null?"未找到目标数字":index));
}
}