二分查找

package foo;



import java.util.Arrays;



public class Main {

    

    /**

     * 二分查找

     * @param key 搜索的目标

     * */

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

        int low = fromIndex;

        int high = toIndex - 1;

        int mid, midVal;

        while (low <= high) {

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

            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.

    }

    

    public static void main(String[] args) throws Exception {

        int[] a = new int[]{1,2,3,4,5};

        System.out.println(binarySearch(a, 0, a.length, 0));

        System.out.println(binarySearch(a, 0, a.length, 1));

        System.out.println(binarySearch(a, 0, a.length, 2));

        System.out.println(binarySearch(a, 0, a.length, 3));

        System.out.println(binarySearch(a, 0, a.length, 4));

        System.out.println(binarySearch(a, 0, a.length, 5));

        System.out.println(binarySearch(a, 0, a.length, 6));

    }

}

折半查找法也称为二分查找法，它充分利用了元素间的次序关系，采用分治策略，可在最坏的情况下用O(log n)完成搜索任务。优点是比较次数少，查找速度快，平均性能好；其缺点是要求待查表为有序表，插入删除困难。因此，折半查找方法适用于不经常变动而查找频繁的有序列表。