binarysearch.java

java编程代码

public class BinarySearch
{
    /**
     Searches the array a for key. If key is not in the array segment, then -1 is
     returned. Otherwise returns an index in the segment such that key == a[index].
     Precondition: a[first] <= a[first + 1]<= ... <= a[last]
    */
    public static int search(int[] a, int first, int last, int key)
    {
        int result = 0; //to keep the compiler happy.

        if (first > last)
            result = -1;
        else
        {
            int mid = (first + last)/2;

            if (key == a[mid])
                result = mid;
            else if (key < a[mid])
                result = search(a, first, mid - 1, key);
            else if (key > a[mid])
                result = search(a, mid + 1, last, key);
        }
        return result;
    }
}