547

Author: ProHiryu

DFS/traditionalDFS/547.md

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+## 547 Friend Circles
+
+#### Description
+
+[link](https://leetcode.com/problems/friend-circles/)
+
+---
+
+#### Solution
+
+- 并查集解法 union find，把所有见到过的全部放到记录set当中，保证主循环中没有见到过的都是新的集合
+
+---
+
+#### Code
+
+> 最坏情况：O(n^2)
+
+```python
+class Solution:
+    def findCircleNum(self, M: List[List[int]]) -> int:
+        if not M:
+            return 0
+        
+        visited = set()
+        N = len(M)
+        res = 0
+        for i in range(N):
+            if i not in visited:
+                res += 1
+                self.dfs(M, i, visited)
+        return res
+    
+    
+    def dfs(self, M, i, visited):
+        visited.add(i)
+        for idx, adj in enumerate(M[i]):
+            if adj == 1 and idx not in visited:
+                self.dfs(M, idx, visited)
+        return 
+```