《算法导论》习题2.3-5 二分搜索 Binary Search

最新推荐文章于 2021-05-05 23:03:23 发布

weixin_30639719

最新推荐文章于 2021-05-05 23:03:23 发布

阅读量153

点赞数

CC 4.0 BY-SA版权

原文链接：http://www.cnblogs.com/wzm-xu/p/4079849.html

地球人都知道“二分查找”，方法也非常简单，但是你能不能在10分钟内写出一个没有bug的程序呢？

知易行难，自己动手写一下试一试吧。

public class BinarySearch {
    public static int search(int [] A,int target,int a, int b)
    {   
        int  middle = (a+b)/2;
        if(a>b)
            return -1;
        else if (A[middle]==target)
            return middle;
        else if (A[middle]< target)
            return search(A,target,middle+1,b);
        else 
            return search(A,target,a,middle-1);
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int A [] = {2,4,6,8,10};
        System.out.println( BinarySearch.search(A,1 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,2 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,3 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,4 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,5 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,6 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,7 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,8 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,9 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,10 , 0, A.length-1) );
        System.out.println( BinarySearch.search(A,11 , 0, A.length-1) );
    }
}

上面的代码看似是没有任何问题了，但是呢，其实还是有一个很微妙的bug，这个bug发生在：

int  middle = (a+b)/2;

这一行。想一下，如果a+b超出了Int型的最大值了呢？？？
所以呢，修正这个bug应该这样：

public class BinarySearch {
    public static int search(int [] A,int target,int a, int b)
    {   
        int  middle = a+(b-a)/2;
        if(a>b)
            return -1;
        else if (A[middle]==target)
            return middle;
        else if (A[middle]< target)
            return search(A,target,middle+1,b);
        else 
            return search(A,target,a,middle-1);
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int A [] = {2,4,6,8,10};
        System.out.println( BinarySearch.search(A,1 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,2 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,3 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,4 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,5 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,6 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,7 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,8 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,9 , 0, A.length-1)  );
        System.out.println( BinarySearch.search(A,10 , 0, A.length-1) );
        System.out.println( BinarySearch.search(A,11 , 0, A.length-1) );
    }
}