839. Similar String Groups

class Solution {
    public int numSimilarGroups(String[] A) {
        Map<String, Set<String>> g = new HashMap<>();
        buildGraph(A, g);
        if (g.size() == 1) return 1;
        // aaaa aaaa
        int count = 0;
        Set<String> visited = new HashSet<>();
        for (String key : g.keySet()) {
            if (visited.contains(key)) continue;
            dfs(visited, key, g);
            count++;
        }

        return count;
    }

    private void dfs(Set<String> visited, String cur, Map<String, Set<String>> g) {
        visited.add(cur);

        if (g.get(cur).size() == 0) return;
        for (String next : g.get(cur)) {
            if (visited.contains(next)) continue;
            dfs(visited, next, g);
        }
    }

    private void buildGraph(String[] A, Map<String, Set<String>> g) {
        for (String s : A) {
            g.put(s, new HashSet<>());
        }

        int n = A.length;
        for (int i = 0; i < n - 1; i++) {
            for (int j = i + 1; j < n; j++) {
                if (isSimilar(A[i], A[j])) {
                    g.get(A[i]).add(A[j]);
                    g.get(A[j]).add(A[i]);
                }
            }
        }
    }

    private boolean isSimilar(String a, String b) {
        // if (a.equals(b)) return true;
        // aaaa aaaa
        int len = a.length();
        int diff = 0;
        for (int i = 0; i < len; i++) {
            if (a.charAt(i) != b.charAt(i)) {
                diff++;
                if (diff > 2) return false;
            }
        }

        return true;
    }
}