二分查找算法

/*

 * 二分查找算法也称为折半搜索、二分搜索，是一种在有序数组中查找某一特定元素的搜索算法。

 * 请注意这种算法是建立在有序数组基础上的。

 * */

public class BinarySearch {

    public static void main(String[] args) {

        int[] a = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18};

        int fromIndex=0;

        int toIndex = a.length-1;

        int key=5;

        System.out.println(binarySearch0(a,fromIndex,toIndex,key));

    }

    

    private static int binarySearch0(int[] a, int fromIndex, int toIndex, int key) {

        int low = fromIndex;

        int high = toIndex-1;

        while (low <= high) {

            int mid = (low + high) >>> 1;

            int midVal = a[mid];

            if (midVal < key)

            low = mid + 1;

            else if (midVal > key)

            high = mid - 1;

            else

            return mid; // key found

        }

        return -(low + 1);  // key not found.

        } 

}