"""

Given an array of distinct integers arr, where arr is sorted in ascending order, return the smallest index i that satisfies arr[i] == i. If there is no such index, return -1.

 

Example 1:

Input: arr = [-10,-5,0,3,7]
Output: 3
Explanation: For the given array, arr[0] = -10, arr[1] = -5, arr[2] = 0, arr[3] = 3, thus the output is 3.
Example 2:

Input: arr = [0,2,5,8,17]
Output: 0
Explanation: arr[0] = 0, thus the output is 0.
Example 3:

Input: arr = [-10,-5,3,4,7,9]
Output: -1
Explanation: There is no such i that arr[i] == i, thus the output is -1.
 

Constraints:

1 <= arr.length < 104
-109 <= arr[i] <= 109

"""

class Solution(object):
    def fixedPoint(self, arr):
        """
        :type arr: List[int]
        :rtype: int
        """
        l, r, result = 0, len(arr)-1, -1
        while l <= r:
            m = l+((r-l)//2)
            if arr[m] < m:
                l = m+1
            elif arr[m] > m:
                r = m-1
            else:
                result, r = m, m-1
        return result
