323. Number of Connected Components in an Undirected Graph

可以繼承 547. Number of Provinces 的做法，先轉換給定的 edges 成一個矩陣。

class Solution:
    def countComponents(self, n: int, edges: List[List[int]]) -> int:
        matrix = [[0]*n for _ in range(n)]
        for edge in edges:
            matrix[edge[0]][edge[1]] = 1
            matrix[edge[1]][edge[0]] = 1

        visited = set()
        count = 0

        def dfs(i):
            for j in range(n):
                if matrix[i][j] == 1 and j not in visited:
                    visited.add(j)
                    dfs(j)

        for i in range(n):
            if i not in visited:
                visited.add(i)
                dfs(i)
                count += 1
        return count

但是這樣的速度太慢了，時間複雜度是 O(n^2) ，空間複雜度也是 O(n^2)。我們可以更一步的精簡，那就是把需要搜索的路徑記錄起來就好了。

記錄的方式可以透過 Hash Table ，key 是島嶼的編號，value 是一個陣列，包含了所有的 edges 。

class Solution:
    def countComponents(self, n: int, edges: List[List[int]]) -> int:
        graph = {}
        visited = set()
        count = 0
        for i in range(n):
            graph[i] = []
        for edge in edges:
            graph[edge[0]].append(edge[1])
            graph[edge[1]].append(edge[0])    
        def dfs(i):
            for j in graph[i]:
                if j not in visited:
                    visited.add(j)
                    dfs(j)
        for i in range(n):
            if i not in visited:
                visited.add(i)
                dfs(i)
                count += 1
        return count

Hash Table 的方式也可以用陣列的取代

graph = [[] for _ in range(n)]
for edge in edges:
    graph[edge[0]].append(edge[1])
    graph[edge[1]].append(edge[0])

Union Find

class Solution:
    def countComponents(self, n: int, edges: List[List[int]]) -> int:
        roots = [i for i in range(n)]
        
        def find(x):
            while x != roots[x]:
                x = roots[x]
            return x
        
        def union(x, y):
            rootX = find(x)
            rootY = find(y)
            if rootX != rootY:
                roots[rootX] = rootY
                return True
            else:
                return False
        
        
        count = n
        for x, y in edges:
            if union(x, y):
                count -= 1
        return count