diff --git a/src/main/java/cn/nextop/rxjava/share/practices/Practice1.java b/src/main/java/cn/nextop/rxjava/share/practices/Practice1.java
index b432fea..7d1afa7 100644
--- a/src/main/java/cn/nextop/rxjava/share/practices/Practice1.java
+++ b/src/main/java/cn/nextop/rxjava/share/practices/Practice1.java
@@ -19,6 +19,9 @@
 import cn.nextop.rxjava.share.util.type.Tuple2;
 import io.reactivex.Observable;
 
+import static cn.nextop.rxjava.share.util.Tuples.of;
+
+
 /**
  * @author Baoyi Chen
  */
@@ -30,6 +33,6 @@ public class Practice1 {
      * 返回值 Observable[(1, "a"), (2, "b"), (3, "c")] 注意index从1开始
      */
     public Observable<Tuple2<Integer, String>> indexable(Observable<String> observable) {
-        throw new UnsupportedOperationException("implementation");
+        return observable.map(i -> of(1, i)).scan((v1, v2) -> of(v1.getV1() + 1, v2.getV2()));
     }
 }
diff --git a/src/main/java/cn/nextop/rxjava/share/practices/Practice2.java b/src/main/java/cn/nextop/rxjava/share/practices/Practice2.java
index 08c7dcd..ca75190 100644
--- a/src/main/java/cn/nextop/rxjava/share/practices/Practice2.java
+++ b/src/main/java/cn/nextop/rxjava/share/practices/Practice2.java
@@ -21,8 +21,11 @@
 import io.reactivex.Observable;
 import io.reactivex.Single;
 
+import java.util.HashMap;
 import java.util.Map;
 
+import static cn.nextop.rxjava.share.util.Tuples.of;
+
 /**
  * @author Baoyi Chen
  */
@@ -34,7 +37,8 @@ public class Practice2 {
      * 返回: Observable[("a", 2), ("b", 1), ("c", 2)]
      */
     public Observable<Tuple2<String, Integer>> wordCount1(Observable<String> words) {
-        throw new UnsupportedOperationException("implementation");
+        // TODO: confused ？
+       return words.groupBy(w -> w).flatMap(g -> g.count().map(c -> of(g.getKey(), c.intValue())).toObservable());
     }
 
     /*
@@ -43,7 +47,7 @@ public Observable<Tuple2<String, Integer>> wordCount1(Observable<String> words)
      * 返回: Single[Map{a=2, b=1, c=2}]
      */
     public Single<Map<String, Integer>> wordCount2(Observable<String> words) {
-        throw new UnsupportedOperationException("implementation");
+        return words.reduceWith(() -> new HashMap<>(), (m, k) -> { m.put(k, m.containsKey(k) ? m.get(k) + 1 : 1); return m; });
     }
 
 }
diff --git a/src/main/java/cn/nextop/rxjava/share/practices/Practice3.java b/src/main/java/cn/nextop/rxjava/share/practices/Practice3.java
index a43bd78..00451f7 100644
--- a/src/main/java/cn/nextop/rxjava/share/practices/Practice3.java
+++ b/src/main/java/cn/nextop/rxjava/share/practices/Practice3.java
@@ -19,6 +19,9 @@
 import io.reactivex.Maybe;
 import io.reactivex.Observable;
 
+import java.util.ArrayList;
+import java.util.List;
+
 /**
  * @author Baoyi Chen
  */
@@ -28,7 +31,7 @@ public class Practice3 {
      * 根据iterate的结果求和
      */
     public Maybe<Integer> sum(Observable<Node> observable) {
-        throw new UnsupportedOperationException("implementation");
+        return this.iterate(observable).reduce((x, y) -> x + y);
     }
 
     /*
@@ -42,7 +45,10 @@ public Maybe<Integer> sum(Observable<Node> observable) {
      * return Observable[4, 3, 6, 7, 5] 顺序无关
      */
     public Observable<Integer> iterate(Observable<Node> observable) {
-        throw new UnsupportedOperationException("implementation");
+        // TODO:  直接定义一个mapper function 应该更decent
+        return observable.flatMapIterable(n -> {
+            List<Node> r = new ArrayList<>(8); this.flat(r, n); return r;
+        }).map(i -> i.value);
     }
 
     public static class Node {
@@ -57,4 +63,9 @@ public Node(Node left, Node right, int value) {
         }
     }
 
+    protected void flat(List<Node> list, Node node) {
+        if (node == null) return;
+        list.add(node); this.flat(list, node.left); this.flat(list, node.right);
+    }
+
 }
diff --git a/src/main/java/cn/nextop/rxjava/share/practices/Practice4.java b/src/main/java/cn/nextop/rxjava/share/practices/Practice4.java
index 33a5804..1b03fe4 100644
--- a/src/main/java/cn/nextop/rxjava/share/practices/Practice4.java
+++ b/src/main/java/cn/nextop/rxjava/share/practices/Practice4.java
@@ -19,6 +19,9 @@
 
 import io.reactivex.Observable;
 
+import static io.reactivex.Observable.just;
+import static io.reactivex.schedulers.Schedulers.io;
+
 
 /**
  * @author Baoyi Chen
@@ -44,7 +47,7 @@ public class Practice4 {
      *
      */
     public Observable<String> runInMultiThread(Observable<String> observable) {
-        throw new UnsupportedOperationException("implementation");
+        return observable.concatMap(o -> just(o).observeOn(io()));
     }
 
 }
diff --git a/src/main/java/cn/nextop/rxjava/share/practices/Practice5.java b/src/main/java/cn/nextop/rxjava/share/practices/Practice5.java
index 1193642..d07538d 100644
--- a/src/main/java/cn/nextop/rxjava/share/practices/Practice5.java
+++ b/src/main/java/cn/nextop/rxjava/share/practices/Practice5.java
@@ -24,6 +24,9 @@
 import java.util.concurrent.TimeUnit;
 import java.util.function.Predicate;
 
+import static io.reactivex.Observable.empty;
+import static io.reactivex.Observable.just;
+
 /**
  * @author Baoyi Chen
  */
@@ -35,7 +38,7 @@ public class Practice5 {
      * return: Single[3]
      */
     public Single<Long> count(Observable<String> source) {
-        throw new UnsupportedOperationException("implementation");
+        return source.reduce(0L, (i, s) -> i + 1);
     }
 
     /*
@@ -44,7 +47,7 @@ public Single<Long> count(Observable<String> source) {
      * return: Observable["a", "b", "c","b", "c", "d"]
      */
     public Observable<String> convert(Observable<List<String>> source) {
-        throw new UnsupportedOperationException("implementation");
+        return source.flatMap(s -> Observable.fromIterable(s));
     }
 
     /*
@@ -53,7 +56,7 @@ public Observable<String> convert(Observable<List<String>> source) {
      * return: Observable["a", "b", "c"]
      */
     public Observable<String> distinct(Observable<String> source) {
-        throw new UnsupportedOperationException("implementation");
+        return source.groupBy(s -> s).map(s -> s.getKey());
     }
 
     /*
@@ -62,7 +65,7 @@ public Observable<String> distinct(Observable<String> source) {
      * return: Observable[3, 4]
      */
     public Observable<Integer> filter(Observable<Integer> source, Predicate<Integer> conditon) {
-        throw new UnsupportedOperationException("implementation");
+        return source.flatMap(s -> conditon.test(s) ? just(s) : empty());
     }
 
     /*
@@ -71,7 +74,7 @@ public Observable<Integer> filter(Observable<Integer> source, Predicate<Integer>
      * return: Maybe[3]
      */
     public Maybe<String> elementAt(Observable<String> source, int index) {
-        throw new UnsupportedOperationException("implementation");
+        return source.skip(index).take(1).firstElement();
     }
 
     /*
@@ -80,7 +83,7 @@ public Maybe<String> elementAt(Observable<String> source, int index) {
      * return: Observable["a", "b", "a", "b"]
      */
     public Observable<String> repeat(Observable<String> source, int count) {
-        throw new UnsupportedOperationException("implementation");
+        return Observable.range(0, count).flatMap(s -> source);
     }
 
     /*
@@ -89,7 +92,7 @@ public Observable<String> repeat(Observable<String> source, int count) {
      * return: Observable["a", "b"]
      */
     public Observable<String> concat(List<Observable<String>> source) {
-        throw new UnsupportedOperationException("implementation");
+        return Observable.fromIterable(source).concatMap(s -> s);
     }
 
     /*
@@ -98,7 +101,7 @@ public Observable<String> concat(List<Observable<String>> source) {
      * return: Observable["a", "b"]
      */
     public Observable<String> merge(List<Observable<String>> source) {
-        throw new UnsupportedOperationException("implementation");
+        return Observable.fromIterable(source).flatMap(s -> s);
     }
 
     /*
@@ -107,7 +110,7 @@ public Observable<String> merge(List<Observable<String>> source) {
      * return: Observable["a", "b", "c"], 每个元素都延迟1秒
      */
     public Observable<String> delayAll(Observable<String> source, long delay, TimeUnit unit) {
-        throw new UnsupportedOperationException("implementation");
+        return source.map(s -> { unit.sleep(delay); return s; });
     }
 
 }