# 323 Number of Connected Components in an Undirected Graph

## 323. [Number of Connected Components in an Undirected Graph](https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/description/)

## 1. Question

Given`n`nodes labeled from`0`to`n - 1`and a list of undirected edges (each edge is a pair of nodes), write a function to find the number of connected components in an undirected graph.

**Example 1:**

```
     0          3
     |          |
     1 --- 2    4
```

Given`n = 5`and`edges = [[0, 1], [1, 2], [3, 4]]`, return`2`.

**Example 2:**

```
     0           4
     |           |
     1 --- 2 --- 3
```

Given`n = 5`and`edges = [[0, 1], [1, 2], [2, 3], [3, 4]]`, return`1`.

**Note:**\
You can assume that no duplicate edges will appear in`edges`. Since all edges are undirected,`[0, 1]`is the same as`[1, 0]`and thus will not appear together in`edges`.

## 2. Implementation

**(1) BFS**

```java
class Solution {
    public int countComponents(int n, int[][] edges) {
        if (n <= 1) {
            return n;
        }

        // BFS
        List<Set<Integer>> adjList = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            adjList.add(new HashSet<>());
        }

        for (int[] edge : edges) {
            adjList.get(edge[0]).add(edge[1]);
            adjList.get(edge[1]).add(edge[0]);
        }

        boolean[] visited = new boolean[n];
        int count = 0;

        for (int i = 0; i < n; i++) {
            if (!visited[i]) {
                ++count;
                Queue<Integer> queue = new LinkedList<>();
                queue.add(i);

                while (!queue.isEmpty()) {
                    int curIndex = queue.remove();
                    visited[curIndex] = true;

                    for (int nextIndex : adjList.get(curIndex)) {
                        if (!visited[nextIndex]) {
                            queue.add(nextIndex);
                        }
                    }
                }
            }
        }
        return count;
    }
}
```

**(2) DFS**

```java
class Solution {
    public int countComponents(int n, int[][] edges) {
        if (n <= 1) {
            return n;
        }

        List<Set<Integer>> adjList = new ArrayList<>();

        for (int i = 0; i < n; i++) {
            adjList.add(new HashSet<>());
        }

        boolean[] visited = new boolean[n];

        for (int[] edge : edges) {
            adjList.get(edge[0]).add(edge[1]);
            adjList.get(edge[1]).add(edge[0]);
        }

        int count = 0;

        for (int i = 0; i < n; i++) {
            if (!visited[i]) {
                ++count;

                searchComponentByDFS(i, adjList, visited);
            }
        }
        return count;
    }

    public void searchComponentByDFS(int node, List<Set<Integer>> adjList, boolean[] visited) {
        visited[node] = true;

        for (int nextNode : adjList.get(node)) {
            if (!visited[nextNode]) {
                searchComponentByDFS(nextNode, adjList, visited);
            }
        }
    }
}
```

**(3) Union Find**

```java
class Solution {
    public int countComponents(int n, int[][] edges) {
        if (n <= 1) {
            return n;
        }

        UnionFind uf = new UnionFind(n);

        for (int[] edge : edges) {
            uf.union(edge[0], edge[1]);
        }
        return uf.count;
    }

    class UnionFind {
        int[] sets;
        int[] size;
        int count;

        public UnionFind(int n) {
            sets = new int[n];
            size = new int[n];
            count = n;

            for (int i = 0; i < n; i++) {
                sets[i] = i;
                size[i] = 1;
            }
        }

        public int find(int node) {
            while (node != sets[node]) {
                node = sets[node];
            }
            return node;
        }

        public void union(int i, int j) {
            int node1 = find(i);
            int node2 = find(j);

            if (node1 == node2) {
                return;
            }

            if (size[node1] < size[node2]) {
                sets[node1] = node2;
                size[node2] += size[node2];
            }
            else {
                sets[node2] = node1;
                size[node1] += size[node2];
            }
            --count;
        }
    }
}
```

## 3. Time & Space Complexity

BFS: 时间复杂度O(n)，空间复杂度O(n)

DFS: 时间复杂度O(n)，空间复杂度O(n)

**Union Find**: 时间复杂度O(n), 空间复杂度O(n)


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