Java集合中二分查找算法实现

Java集合中二分查找算法实现

Arrays.binarySearch实现了对有序数组特定区间的二分查找，虽然我们觉得很简单，但是阅读源码的确能看到实现这些库的优秀技巧，总是在追求完美和高效。

值得学习的地方有：

（1）边界检查；

（2）求中位数的时候使用位移操作，而不是 x/2;

（3）如果查找的元素不在数组中，通过返回值昭示了应该插入的位置，而不是直接返回-1；

public static int binarySearch(int[] a, int fromIndex, int toIndex,

                                   int key) {

        rangeCheck(a.length, fromIndex, toIndex);

        return binarySearch0(a, fromIndex, toIndex, key);

    }

    // Like public version, but without range checks.

    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.

    }

 

同样在Collections里面同样有类似的辅助函数，不过是利用迭代器取得特定位置的元素。

public static 
    int binarySearch(List> list, T key) {
        if (list instanceof RandomAccess || list.size()< BINARYSEARCH_THRESHOLD)
            return Collections. indexedBinarySearch(list, key);
        else
            return Collections. iteratorBinarySearch(list, key);
    }


private static 
    int indexedBinarySearch(List> list, T key ) {
        int low = 0;
        int high = list.size()-1;

        while (low <= high) {
            int mid = ( low + high) >>> 1;
            Comparable midVal = list.get( mid);
            int cmp = midVal.compareTo( key);

            if ( cmp < 0)
                low = mid + 1;
            else if ( cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }

    private static 
    int iteratorBinarySearch(List> list, T key )
    {
        int low = 0;
        int high = list.size()-1;
        ListIterator> i = list.listIterator();

        while (low <= high) {
            int mid = ( low + high) >>> 1;
            Comparable midVal = get(i , mid );
            int cmp = midVal.compareTo( key);

            if ( cmp < 0)
                low = mid + 1;
            else if ( cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }

 /**
     * Gets the ith element from the given list by repositioning the specified
     * list listIterator.
     */
    private static  T get(ListIterator i, int index) {
        T obj = null;
        int pos = i.nextIndex();
        if (pos <= index) {
            do {
                obj = i.next();//链表移动
            } while ( pos++ < index);
        } else {
            do {
                obj = i.previous();
            } while (-- pos > index);
        }
        return obj;
    }