二分搜索

int binary_search ( int n , int low , int high )
{
    int mid = ( low + high ) >> 1 ;    //下标大会溢出

    while ( low <= high )
    {
        if ( a [ mid ] > n )
            high = mid - 1 ;
        else if ( a [ mid ] < n )
            low = mid + 1 ;
        else
            return mid ;
        mid = ( low + high ) >> 1 ;
    }

    return -1 ;
}