习题1.8 二分查找 (20 分)

    Position BinarySearch(List L, ElementType X){
        Position Left,Right,Mid;
        Left=1;
        Right=L->Last;
        while(Left<=Right){
            Mid=(Left+Right)/2;
            if(X==L->Data[Mid])
                return Mid;
            else if(X>L->Data[Mid]){
                Left=Mid+1;
            }
            else
                Right=Mid-1;
        }
        return NotFound;
    }