题目地址：https://leetcode-cn.com/problems/number-of-connected-components-in-an-undirected-graph/

题目描述

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:

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

     0          3
     |          |
     1 --- 2    4 

Output: 2

Example 2:

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

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

Output:  1

Note:

Youcan 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.

题目大意

给定编号从 0 到 n-1 的 n 个节点和一个无向边列表（每条边都是一对节点），请编写一个函数来计算无向图中连通分量的数目。

解题方法

并查集

看到求联通分量的题，一般都可以用并查集。比如1101. The Earliest Moment When Everyone Become Friendsopen in new window。

只要把并查集背下来，这个题目基本直接写上去就好了。

C++代码如下：

class Solution {
public:
    int countComponents(int n, vector<vector<int>>& edges) {
        map_ = vector<int>(n, 0);
        components = n;
        for (int i = 0; i < n; ++i) {
            map_[i] = i;
        }
        for (vector<int>& edge : edges) {
            uni(edge[0], edge[1]);
        }
        return components;
    }
    int find(int a) {
        if (a == map_[a])
            return a;
        return find(map_[a]);
    }
    void uni(int a, int b) {
        int pa = find(a);
        int pb = find(b);
        if (pa == pb)
            return;
        map_[pa] = pb;
        components --;
    }
private:
    vector<int> map_;
    int components;
};

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