参考代码：

def binarySearch (arr, l, r, x): 
    if r >= l: 
        mid = int(l + (r - l)/2)
        if arr[mid] == x: 
            return mid 
        elif arr[mid] > x: 
            return binarySearch(arr, l, mid-1, x) 
        else: 
            return binarySearch(arr, mid+1, r, x) 
    else: 
        return -1