アリインタビューZhenti：バイナリサーチアルゴリズム

実際には、この問題は難しくありませんが、異常な状態であればわからない多くの学生のために問題があるだろうので、それは、非常に実用的です。

ルックは、JDKバイナリ検索バーが付属しています

/**
     * Searches a range of
     * the specified array of longs for the specified value using the
     * binary search algorithm.
     * The range must be sorted (as
     * by the {@link #sort(long[], int, int)} method)
     * prior to making this call.  If it
     * is not sorted, the results are undefined.  If the range contains
     * multiple elements with the specified value, there is no guarantee which
     * one will be found.
     *
     * @param a the array to be searched
     * @param fromIndex the index of the first element (inclusive) to be
     *          searched
     * @param toIndex the index of the last element (exclusive) to be searched
     * @param key the value to be searched for
     * @return index of the search key, if it is contained in the array
     *         within the specified range;
     *         otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>.  The
     *         <i>insertion point</i> is defined as the point at which the
     *         key would be inserted into the array: the index of the first
     *         element in the range greater than the key,
     *         or <tt>toIndex</tt> if all
     *         elements in the range are less than the specified key.  Note
     *         that this guarantees that the return value will be &gt;= 0 if
     *         and only if the key is found.
     * @throws IllegalArgumentException
     *         if {@code fromIndex > toIndex}
     * @throws ArrayIndexOutOfBoundsException
     *         if {@code fromIndex < 0 or toIndex > a.length}
     * @since 1.6
     */
    public static int binarySearch(long[] a, int fromIndex, int toIndex,
                                   long key) {
        rangeCheck(a.length, fromIndex, toIndex);
        return binarySearch0(a, fromIndex, toIndex, key);
    }

 /**
     * Checks that {@code fromIndex} and {@code toIndex} are in
     * the range and throws an exception if they aren't.
     */
    private static void rangeCheck(int arrayLength, int fromIndex, int toIndex) {
        if (fromIndex > toIndex) {
            throw new IllegalArgumentException(
                    "fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")");
        }
        if (fromIndex < 0) {
            throw new ArrayIndexOutOfBoundsException(fromIndex);
        }
        if (toIndex > arrayLength) {
            throw new ArrayIndexOutOfBoundsException(toIndex);
        }
    }

// Like public version, but without range checks.
    private static int binarySearch0(long[] a, int fromIndex, int toIndex,
                                     long key) {
        int low = fromIndex;
        int high = toIndex - 1;

        while (low <= high) {
            int mid = (low + high) >>> 1;
            long 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.
    }