Number of Connected Components in an Undirected Graph -- LeetCode

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

.

思路：并查集(Union find)

class Solution {
public:
int getFather(vector<int>& father, int i) {
if (father[i] == i) return i;
father[i] = getFather(father, father[i]);
return father[i];
}
void merge(vector<int>& father, int i, int j) {
int fatherI = getFather(father, i);
int fatherJ = getFather(father, j);
father[fatherJ] = fatherI;
}
int countComponents(int n, vector<pair<int, int>>& edges) {
vector<int> father;
for (int i = 0; i < n; i++) father.push_back(i);
for (int i = 0, n = edges.size(); i < n; i++)
merge(father, get<0>(edges[i]), get<1>(edges[i]));
unordered_set<int> unions;
for (int i = 0; i < n; i++) unions.insert(getFather(father, i));
return unions.size();
}
};