Find Latest Group of Size M — Python

# Time:  O(n)
# Space: O(n)

class Solution(object):
    def findLatestStep(self, arr, m):
        """
        :type arr: List[int]
        :type m: int
        :rtype: int
        """
        if m == len(arr):
            return m
        length = [0]*(len(arr)+2)
        result = -1
        for i, x in enumerate(arr):
            left, right = length[x-1], length[x+1]
            if left == m or right == m:
                result = i
            length[x-left] = length[x+right] = left+right+1
        return result