Least index matches its value

Binary search:
有兩種寫法， <=和< ，l永遠都是mid+1，只有r會不一樣

"""
Given a sorted array arr of distinct integers,
write a function indexEqualsValueSearch that returns the lowest index i for which arr[i] == i.
Return -1 if there is no such index. Analyze the time and space complexities of your solution and explain its correctness.
"""


def indexEqualsValueSearch(arr):

    l = 0
    r = len(arr) - 1

    while l <= r:

        mid = l + (r-l)/2

        if mid > arr[mid]:
            l = mid + 1
        elif mid < arr[mid]:
            r = mid - 1
        else:
            if mid and arr[mid-1] == mid - 1:
                r = mid - 1
            else:
                return mid
    return -1

arr = [-9,0,2,5]
print indexEqualsValueSearch(arr)