diff --git a/algs4/stopwatch.py b/algs4/stopwatch.py
new file mode 100644
index 0000000..cbfd7ad
--- /dev/null
+++ b/algs4/stopwatch.py
@@ -0,0 +1,10 @@
+import time
+
+
+class StopWatch:
+
+    def __init__(self):
+        self.start = time.time()
+
+    def elapsed_time(self):
+        return round(time.time - self.start, 2)
diff --git a/algs4/uf.py b/algs4/uf.py
new file mode 100644
index 0000000..87fd41c
--- /dev/null
+++ b/algs4/uf.py
@@ -0,0 +1,72 @@
+class UF:
+
+    def __init__(self, n):
+        self.count = n
+        self.id = list(range(n))
+        self.sz = [1] * n
+
+    def connected(self, p, q):
+        return self.find(p) == self.find(q)
+
+    # quick-find
+    # def find(self, p):
+    #     return self.id[p]
+
+    # def union(self, p, q):
+    #     pId = self.find(p)
+    #     qId = self.find(q)
+    #     if pId == qId:
+    #         return
+    #     for index, i in enumerate(self.id):
+    #         if i == pId:
+    #             self.id[index] = qId
+    #     self.count -= 1
+
+    # quick-union
+    # def find(self, p):
+    #     while self.id[p] != p:
+    #         p = self.id[p]
+    #     return p
+
+    # def union(self, p, q):
+    #     pId = self.find(p)
+    #     qId = self.find(q)
+    #     if pId == qId:
+    #         return
+    #     self.id[pId] = qId
+    #     self.count -= 1
+
+    # weighted quick-union
+    def find(self, p):
+        while self.id[p] != p:
+            self.id[p] = self.id[self.id[p]]  # path compression
+            p = self.id[p]
+        return p
+
+    def union(self, p, q):
+        pId = self.find(p)
+        qId = self.find(q)
+        if pId == qId:
+            return
+        if self.sz[pId] < self.sz[qId]:
+            self.id[pId] = qId
+            self.sz[qId] += self.sz[pId]
+        else:
+            self.id[qId] = pId
+            self.sz[pId] += self.sz[qId]
+        self.count -= 1
+
+
+if __name__ == '__main__':
+    import sys
+
+    n = int(sys.stdin.readline())
+    uf = UF(n)
+    for line in sys.stdin:
+        p, q = [int(i) for i in line.split()]
+        if (uf.connected(p, q)):
+            continue
+        else:
+            uf.union(p, q)
+            print("%s %s" % (p, q))
+    print(uf.count, "components")