[NeetCode 150] Count Connected Components

Count Connected Components

There is an undirected graph with n nodes. There is also an edges array, where edges[i] = [a, b] means that there is an edge between node a and node b in the graph.

Return the total number of connected components in that graph.

Example 1:

Input:
n=3
edges=[[0,1], [0,2]]

Output:
1

Example 2:

Input:
n=6
edges=[[0,1], [1,2], [2,3], [4,5]]

Output:
2

Constraints:

1 <= n <= 100
0 <= edges.length <= n * (n - 1) / 2

Solution

All we need to do is recording whether a node has been visited and keep applying DFS on unvisited nodes. A connected component will all be marked as visited in one-time DFS so we just need to count how many times of DFS we have launched.

Code

class Solution:
    def countComponents(self, n: int, edges: List[List[int]]) -> int:
        vis_flag = [False] * n
        graph_map = {i: [] for i in range(n)}
        for edge in edges:
            graph_map[edge[0]].append(edge[1])
            graph_map[edge[1]].append(edge[0])
        def dfs(node):
            vis_flag[node] = True
            for nxt in graph_map[node]:
                if not vis_flag[nxt]:
                    dfs(nxt)
        answer = 0
        for i in range(n):
            if not vis_flag[i]:
                dfs(i)
                answer += 1
        return answer