323. Number of Connected Components in an Undirected Graph

You have a graph of n nodes. You are given an integer n and an array edges where edges[i] = [ai, bi] indicates that there is an edge between ai and bi in the graph.

Return the number of connected components in the graph.

 

Example 1:

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

Example 2:

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

class Solution {
    public int countComponents(int n, int[][] edges) {
    int[] roots = new int[n];
    for(int i = 0; i < n; i++) roots[i] = i; 

    for(int[] e : edges) {
        int root1 = find(roots, e[0]);
        int root2 = find(roots, e[1]);
        if(root1 != root2) {      
            roots[root1] = root2;  // union
            n--;
        }
    }
    return n;
}

public int find(int[] roots, int id) {
    while(roots[id] != id) {
        roots[id] = roots[roots[id]];  // optional: path compression
        id = roots[id];
    }
    return id;
}
}