diff --git a/algs4/heap.py b/algs4/heap.py
index e4e7962..01bffb2 100644
--- a/algs4/heap.py
+++ b/algs4/heap.py
@@ -52,11 +52,20 @@ def sort(cls, arr):
 
         return arr
 
+    @classmethod
+    def is_sorted(cls, arr):
+        for i in range(1, len(arr)):
+            if arr[i] < arr[i-1]:
+                return False
+        return True
+
 
 if __name__ == '__main__':
     import sys
 
+    items = []
     for line in sys.stdin:
-        items = line.split()
-        print('     items: ', items)
-        print('sort items: ', Heap.sort(items))
+        items.extend(line.split())
+    print('     items: ', items)
+    print('sort items: ', Heap.sort(items))
+    assert Heap.is_sorted(items)
diff --git a/algs4/insertion.py b/algs4/insertion.py
index 5382837..622a331 100644
--- a/algs4/insertion.py
+++ b/algs4/insertion.py
@@ -37,11 +37,20 @@ def sort(cls, arr):
                 j -= 1
         return arr
 
+    @classmethod
+    def is_sorted(cls, arr):
+        for i in range(1, len(arr)):
+            if arr[i] < arr[i-1]:
+                return False
+        return True
+
 
 if __name__ == '__main__':
     import sys
 
+    items = []
     for line in sys.stdin:
-        items = line.split()
-        print('     items: ', items)
-        print('sort items: ', Insertion.sort(items))
+        items.extend(line.split())
+    print('     items: ', items)
+    print('sort items: ', Insertion.sort(items))
+    assert Insertion.is_sorted(items)
diff --git a/algs4/merge.py b/algs4/merge.py
index 317aeb6..003d9dd 100644
--- a/algs4/merge.py
+++ b/algs4/merge.py
@@ -51,7 +51,7 @@ def mergesort(cls, arr, lo, hi):
         if lo >= hi:
             return
 
-        mid = (lo + hi) / 2
+        mid = (lo + hi) // 2
         cls.mergesort(arr, lo, mid)
         cls.mergesort(arr, mid + 1, hi)
         cls.merge(arr, lo, mid, hi)
@@ -61,11 +61,20 @@ def mergesort(cls, arr, lo, hi):
     def sort(cls, arr):
         return cls.mergesort(arr, 0, len(arr) - 1)
 
+    @classmethod
+    def is_sorted(cls, arr):
+        for i in range(1, len(arr)):
+            if arr[i] < arr[i-1]:
+                return False
+        return True
+
 
 if __name__ == '__main__':
     import sys
 
+    items = []
     for line in sys.stdin:
-        items = line.split()
-        print('     items: ', items)
-        print('sort items: ', Merge.sort(items))
+        items.extend(line.split())
+    print('     items: ', items)
+    print('sort items: ', Merge.sort(items))
+    assert Merge.is_sorted(items)
diff --git a/algs4/selection.py b/algs4/selection.py
index 3ba0f0b..183e7cb 100644
--- a/algs4/selection.py
+++ b/algs4/selection.py
@@ -36,11 +36,20 @@ def sort(cls, arr):
             arr[i], arr[minIndex] = arr[minIndex], arr[i]
         return arr
 
+    @classmethod
+    def is_sorted(cls, arr):
+        for i in range(1, len(arr)):
+            if arr[i] < arr[i-1]:
+                return False
+        return True
+
 
 if __name__ == '__main__':
     import sys
 
+    items = []
     for line in sys.stdin:
-        items = line.split()
-        print('     items: ', items)
-        print('sort items: ', Selection.sort(items))
+        items.extend(line.split())
+    print('     items: ', items)
+    print('sort items: ', Selection.sort(items))
+    assert Selection.is_sorted(items)
diff --git a/algs4/shell.py b/algs4/shell.py
index c761647..aa151b9 100644
--- a/algs4/shell.py
+++ b/algs4/shell.py
@@ -1,7 +1,7 @@
 """
 Sorts a sequence of strings from standard input using shell sort.
 
- 
+
 % more tiny.txt
 
 S O R T E X A M P L E
@@ -10,7 +10,7 @@
 
 A E E L M O P R S T X                 [ one string per line ]
 
-  
+
 
 % more words3.txt
 
@@ -29,7 +29,7 @@ class Shell:
     def sort(cls, arr):
         N = len(arr)
         h = 1
-        while h < N / 3:
+        while h < N // 3:
             h = 3 * h + 1  # 1, 4, 13, 40...
 
         while h >= 1:
@@ -40,14 +40,23 @@ def sort(cls, arr):
                         break
                     arr[j], arr[j - h] = arr[j - h], arr[j]
                     j -= h
-            h /= 3
+            h //= 3
         return arr
 
+    @classmethod
+    def is_sorted(cls, arr):
+        for i in range(1, len(arr)):
+            if arr[i] < arr[i-1]:
+                return False
+        return True
+
 
 if __name__ == '__main__':
     import sys
 
+    items = []
     for line in sys.stdin:
-        items = line.split()
-        print('     items: ', items)
-        print('sort items: ', Shell.sort(items))
+        items.extend(line.split())
+    print('     items: ', items)
+    print('sort items: ', Shell.sort(items))
+    assert Shell.is_sorted(items)