Given a 2-D array of 0s and 1s, find islands in it. An Island is 1s together

class Solution(object):
    def count_islands(self,arr):
        island_count = 0

        for i in range(len(arr)):
            for j in range(len(arr[i])):
                if arr[i][j]:
                    self._dfs_util(arr,i,j)
                    island_count+=1
        return island_count

    def _dfs_util(self,arr,i,j):
        if i < len(arr) and i>=0 and j>=0 and j<len(arr[i]) and arr[i][j]==1:

            arr[i][j] = 0

            self._dfs_util(arr,i-1,j)
            self._dfs_util(arr,i+1,j)
            self._dfs_util(arr,i,j-1)
            self._dfs_util(arr,i,j+1)

Time Complexity is O(m*n) if shape of arr is m,n