Implementation 1:
    private static class UnionFind<T> {
        private HashMap<T, T> f = new HashMap<>();
        
        T find(T x) {
            T y = f.getOrDefault(x, x);
            
            if(y != x) {
                y = find(y);
                f.put(x, y);
            }
            return y;
        }
        
        void union(T x, T y) {
            f.put(find(x), find(y));
        }
    }

Implementation 2:
class Solution {
    int[] parent;
    int n;
    
    void union(int x, int y) {
        int px = find(x);
        int py = find(y);
        
        if(px != py) {
            parent[px] = py;
            n--;
        }
    }
    
    int find(int x) {
        if(parent[x] == x) return x;
        
        parent[x] = find(parent[x]);
        
        return parent[x];
    }
}