leetcode Number of Connected Components in an Undirected Graph

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

.

union-find问题，思路是每个点记录其父节点的索引，每次union时找到两点对应的根节点，将根节点相连，最后遍历数组，找到所有的根节点即可：

private int[] res;
public int countComponents(int n, int[][] edges) {
res=new int
;
for(int i=0;i<n;i++)
res[i]=i;
union(edges);
int k=0;
for(int i=0;i<res.length;i++){
if(res[i]==i) k++;
}
return k;
}
public void union(int[][] edges){
for(int[] edge:edges){
int i=FindRoot(res,edge[0]);
int j=FindRoot(res,edge[1]);
res[i]=j;
}
}
public int FindRoot(int[] res,int k){
while(k!=res[k]) k=res[k];
return k;

}