refactor(datastructures, union-find): add type hints and doc comments

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

Author: BrianLusina

datastructures/sets/union_find/__init__.py

@@ -46,17 +46,29 @@ def get_count(self) -> int:
 
 
 class UnionFind:
-    def __init__(self, size):
+    """A minimal Union-Find data structure with path compression."""
+    
+    def __init__(self, size: int):
+        """Initializes the data structure with 'size' elements."""
+        if size <= 0:
+            raise ValueError("Size must be a positive integer.")
         self.parent = list(range(size))
 
-    def find(self, x):
+    def find(self, x: int) -> int:
+        """Finds the representative (root) of the set containing element 'x'."""
         if self.parent[x] != x:
             # Path compression
             self.parent[x] = self.find(self.parent[x])
         return self.parent[x]
 
-    def union(self, x, y):
+    def union(self, x: int, y: int) -> bool:
+        """
+        Merges the sets containing elements 'x' and 'y'.
+        Returns True if a merge occurred, False if already in same set.
+        """
         root_x = self.find(x)
         root_y = self.find(y)
         if root_x != root_y:
-            self.parent[root_y] = root_x
+            self.parent[root_y] = root_x
+            return True
+        return False