LeetCode323 Number of Connected Components in an Undirected Graph

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

 0          3
 |          |
 1 --- 2    4 

Output: 2

it is quite easy to understtandi this problem.

now, how to solve it?
if you really think about this, it is a classic-classic union find problem

just union and find. recite it like presort/postorder/inorder.levelirder

Input: n = 5 and edges = [[0, 1], [1, 2], [3, 4]]
class Solution {
    public int countComponents(int n, int[][] edges) {
        int[] roots = new int[n];
        for (int i = 0; i < n; i++) { //initialize every node as its father node
            roots[i] = i;
        }
        
        for (int[] e:edges) { //for every edges
            int root1 = find(roots, e[0]);
            int root2 = find(roots, e[1]);
            if (root1 != root2) {
                roots[root1] = root2; //attach root1 to root2
                n--;
            }
        }
        return n;
        
    }
    
    private int find(int[] roots, int id) {
        int oid = id;
        while (roots[id] != id) {
            id = roots[id]; //go up to previous level
        }//and the id will final locates at where the untimate root of oid 
        roots[oid] = id; //and finally, the id node is directed to the aultimate root
        return id;
    }
}