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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