547. Friend Circles

# time O(N^2) space O(N)
class Solution:
    def findCircleNum(self, M: List[List[int]]) -> int:
        size = len(M)
        cnt  = 0
        visited  = [False] * size
        for i in range(size):
            if(not visited[i]):
                self.dfs(M,visited,i)
                cnt += 1
        return cnt
    
    def dfs(self,M,visited, person):
        for other in range(len(M)):
            # found another person in current person's friend cycle 
            if M[person][other] == 1 and not visited[other]:
                visited[other] = True
                self.dfs(M,visited,other) # start a new DFS based on the reached friend