二分查找

/**

     * 使用 while 查找值所在位置

     * @param arr 目标数组，必须是排序后的

     * @param value 查找目标值

     * @return 位置

     */

    private static Integer binarySearch1( Integer[] arr, Integer value ){

        int low = 0;

        int high = arr.length-1;

        while ( low<high ){

            int middle = (low+high)/2;

            if ( value == arr[middle] ){

                return middle;

            }

            if ( value < arr[middle] ){

                high = middle-1;

            }

            if ( value > arr[middle] ){

                low = middle+1;

            }

        }

        return -1;

    }

    /**

     * 递归查询

     * @param arr 目标数组，必须是排序后的

     * @param value 查找目标值

     * @param startIndex 起始位置

     * @param endIndex 结束位置

     * @return 位置

     */

    private static Integer binarySearch2( Integer[] arr,Integer value,Integer startIndex,Integer endIndex ){

        int middleIndex = (startIndex+endIndex)/2;

        if ( value<arr[startIndex] || value>arr[endIndex] ){

            return -1;

        }

        if ( value<arr[middleIndex] ){

            return binarySearch2( arr,value,startIndex,middleIndex-1 );

        }

        if ( value>arr[middleIndex] ){

            return binarySearch2( arr,value,middleIndex+1,endIndex );

        }else {

            return middleIndex;

        }

    }

    public static void main(String[] args) {

        Integer[] arr1 = new Integer[]{2,14,65,23,41,12,54,23,56,20};

        Arrays.sort(arr1);

        System.out.println( Arrays.asList(arr1).toString() );

        Integer result1 = binarySearch1(arr1,54);

        System.out.println(result1);

        Integer[] arr2 = new Integer[]{2,14,65,23,41,12,54,23,56,20};

        Arrays.sort(arr2);

        System.out.println( Arrays.asList(arr2).toString() );

        Integer result2 = binarySearch2(arr2,56,0,arr2.length-1);

        System.out.println(result2);

    }