JAVA实现二分查找

二分查找又称折半查找、二叉查找，它是一种效率较高的查找方法。

前提

给定一已排好序的n个元素a[0 : n-1]，现要在这n个元素中找出一特定元素x。

算法 思想

首先，将表中间位置记录的关键字与查找关键字比较，如果两者相等，则查找成功；否则利用中间位置记录将表分成前、后两个子表，如果中间位置记录的关键字大 于查找关键字，则进一步查找前一子表，否则进一步查找后一子表。重复以上过程，直到找到满足条件的记录，使查找成功，或直到子表不存在为止，此时查找不成功。

public class BinarySearch {

    /***
     * 
     * @param a 已排好序的数组
     * @param dst 需要查找的目标值
     * @return 查找到目标值返回目标值在数组中的位置，没有超重到目标值返回-1
     */
    public int binarySearch(int[] a, int dst) {
        
        int flag = 0;
        //先判断数组是顺序还是倒序
        if (a[0] > a[a.length - 1]) {
            System.out.println("array is reverse order");
            flag = 1;  //数组为倒序
        }
        
        int mid = -1;
        int low = 0;
        int high = a.length - 1;
        if (0 == flag) {
            while (low <= high) {
                mid = low + (high-low)/2; //此处不用(low + high)/2是防止low + high值溢出
                if (a[mid] == dst) {
                    printSearchResult(a,dst,mid);
                    return mid;
                } else if (dst < a[mid]) {
                    high = mid - 1;
                } else {
                    low = mid + 1;
                }
            }
        } else {
            while (low <= high) {
                mid = low + (high-low)/2; //此处不用(low + high)/2是防止low + high值溢出
                if (a[mid] == dst) {
                    printSearchResult(a,dst,mid);
                    return mid;
                } else if (dst < a[mid]) {
                    low = mid + 1;
                } else {
                    high = mid - 1;
                }
            }
        }
        
        
        System.out.println("not find the num " + dst + " in the array");
        return -1;
        
    }
    
    /***
     * 打印查找结果
     * @param a 数组
     * @param dst 查找的目标值
     * @param position 目标值在数组中的位置
     */
    private void printSearchResult(int[] a,int dst, int position) {
        String tmp = "find the num " + dst + " in the array {";
        for (int i = 0; i < a.length - 2; i++) {
            tmp += a[i] + ",";
        }
        tmp += a[a.length - 1] + "}, position is " + position;
        System.out.println(tmp);
    }
    
    public static void main(String[] args) {
        int a[] = {1,3,5,7,8,9,10,12,15,17};
        BinarySearch binarySearch = new BinarySearch();
        binarySearch.binarySearch(a,17);
        binarySearch.binarySearch(a, 8);
        int b[] = {17,13,11,9,7,6,4,2,1};
        binarySearch.binarySearch(b,17);
        binarySearch.binarySearch(b, 4);
        binarySearch.binarySearch(b, 1);
    }
    
}

public class BinarySearch {
    public static void main(String[] args) {
        int[] arys = {1, 3, 6, 11, 35, 36, 45, 66, 75};
        System.out.println(binarySearch(arys, 6, 0, arys.length - 1));
    }

    private static int binarySearch(int[] arys, int element, int begin, int end) {
        // TODO Auto-generated method stub
        if (begin <= end) {
            int mid = (begin + end) >> 1;
            if (arys[mid] == element)
                return mid;
            else if(arys[mid] > element) {
                return binarySearch(arys, element, 0, mid - 1);
            } else {
                return binarySearch(arys, element, mid + 1, end);
            }
        } else
        return -1;
    }
}