diff --git a/challenge_10/python/shaowen/solution.py b/challenge_10/python/shaowen/solution.py
index e7902ef6b..980ab96dd 100644
--- a/challenge_10/python/shaowen/solution.py
+++ b/challenge_10/python/shaowen/solution.py
@@ -6,16 +6,16 @@ def solution(strr):
     """check if all brackets closed properly"""
     
     closerof = {'{':'}', '[':']','(':')'}
-    Start = ('[','{','(')
-    Closer = (']','}',')')
+    START = ('[','{','(')
+    CLOSE = (']','}',')')
     stack = []
     
     for i in strr:
         # push into stack, if new bracket found
-        if i in Start:
+        if i in START:
             stack.append(i)
         # pop out if proper closer found, else False
-        elif i in Closer:
+        elif i in CLOSE:
             if closerof[stack[-1]] == i:
                 stack.pop()
             else:
diff --git a/challenge_11/python/shaowen/READE.md b/challenge_11/python/shaowen/READE.md
new file mode 100644
index 000000000..d8d5a14da
--- /dev/null
+++ b/challenge_11/python/shaowen/READE.md
@@ -0,0 +1,41 @@
+###Challenge 11
+
+Language Version: Python 2.7.12
+
+
+### Solution
+
+Algorithms ref:
+
+http://www.algolist.net/Data_structures/Binary_search_tree/Removal
+
+### How to run
+
+$ python bst.py
+
+Please type in a list of node integer, seperated by space.
+
+5 2 1 3 12 9 21 19 36
+
+Original bst list: 
+
+[5, 2, 1, 3, 12, 9, 21, 19, 36]
+
+
+Please type in key(s) you want to delete:
+
+(you could type in one int, or a list of int like following, if the key is not in bst, you will also be notified.)
+
+5 7 2 19
+
+key 7 not found.
+
+New bst list:
+
+[9, 3, 1, 12, 21, 36]
+
+
+
+###Testing
+
+$ python unit_test.py
diff --git a/challenge_11/python/shaowen/bst.py b/challenge_11/python/shaowen/bst.py
new file mode 100644
index 000000000..440c19dac
--- /dev/null
+++ b/challenge_11/python/shaowen/bst.py
@@ -0,0 +1,189 @@
+# -*- coding: utf-8 -*-
+# @author: Shaowen Liu
+
+class Node():
+    
+    def __init__(self, key, left=None, right=None, parent=None):
+        self.key = key
+        self.left = left
+        self.right = right
+        self.parent = parent
+        
+    def __str__(self):
+        return str(self.key)
+        
+    def has_any_child(self):
+        return self.left or self.right
+    
+    def the_only_child(self):
+        """if the node has only one child, return the child
+        else, return False
+        """
+        if self.left and (not self.right):
+            return self.left
+        elif self.right and (not self.left):
+            return self.right
+        else:
+            return False
+    
+    def is_left_child(self):
+        return bool(self.parent.left == self)
+    
+    def is_right_child(self):
+        return bool(self.parent.right == self)
+            
+class BST():
+    
+    def __init__(self, root = None, size = 0):
+        self.root = root
+        self.size = size
+    
+    def _insert(self, key, current_node):
+        
+        if key == current_node.key:
+            raise IndexError('Find duplicated key.')
+        elif key > current_node.key:
+            if current_node.right:
+                self._insert(key, current_node.right)
+            else:
+                current_node.right = Node(key, parent = current_node)
+        elif key < current_node.key:
+            if current_node.left:
+                self._insert(key, current_node.left)
+            else:
+                current_node.left = Node(key, parent = current_node)
+        
+    def insert(self, key):
+        """insert new key into bst"""
+        # if bst does not exist, make new node the root
+        if not self.root:
+            self.root = Node(key, parent = None)
+        # else insert the node into bst recercively
+        else:
+            self._insert(key, self.root)
+        self.size += 1
+    
+    def _has_key(self, key, current_node):
+        
+        if key == current_node.key:
+            return current_node
+        # lookup right branch
+        elif key > current_node.key:
+            return self._has_key(key, current_node.right)
+        # lookup left branch
+        elif key < current_node.key:
+            return self._has_key(key, current_node.left)
+        
+    def has_key(self, key):
+        """lookup key in bst,
+        return node if key found, else False
+        """
+        if not self.root: 
+            return False
+        else:
+            return self._has_key(key, self.root)
+    
+    def smallest_key(self, current_node):
+        """start from current_node,
+        search the node with the minimum key in bst
+        """
+        if not current_node:
+            raise IndexError('this is an empty (sub)tree')
+        
+        if current_node.left:
+            return self.smallest_key(current_node.left)
+        else:
+            return current_node
+        
+    def _delete(self, removed_node):
+        
+        # if removed_node has no child
+        if not removed_node.has_any_child():
+            # removed node is a leaf
+            if removed_node.parent:
+                if removed_node.is_left_child():
+                    removed_node.parent.left = None
+                else:
+                    removed_node.parent.right = None
+                self.size -= 1
+            # removed node is root 
+            else:
+                self.root = None
+                self.size = 0
+                
+        # removed node has only one child
+        elif removed_node.the_only_child():
+            if removed_node.parent:
+                if removed_node.is_left_child():
+                    removed_node.parent.left = removed_node.the_only_child()
+                    removed_node.the_only_child().parent = removed_node.parent
+                else:
+                    removed_node.parent.right = removed_node.the_only_child()
+                    removed_node.the_only_child().parent = removed_node.parent
+            # removed node is root
+            else:
+                self.root = self.root.the_only_child()
+        
+        # removed node has two children
+        else:
+            min_node = self.smallest_key(removed_node.right)
+            removed_node.key = min_node.key
+            self._delete(min_node) 
+            
+    def delete(self, key):
+        """delete key from bst"""
+        removed_node = self.has_key(key)
+        # if bst has the key, kick off delete process
+        if removed_node:
+            self._delete(removed_node)
+        else:
+            raise ValueError('key not found.')
+    
+    def _pre_order(self, current_node):
+        
+        arr = []
+        arr.append(current_node.key)
+        if current_node.left:
+            arr += self._pre_order(current_node.left)
+        if current_node.right:
+            arr += self._pre_order(current_node.right)
+        return arr
+        
+    def pre_order_print(self):
+        """print the tree by pre_order traversal"""
+        if self.root:
+            return self._pre_order(self.root)
+        else:
+            return []
+
+def build_bst(arr):
+    """build a bst from an array"""
+    bst = BST()
+    for i in arr:
+        bst.insert(i)
+    return bst
+
+if __name__ == '__main__':
+    #user input
+    node_arr = raw_input('Please type in a list of node integer, seperated by space.\n')
+    # insert nodes
+    bst = build_bst(map(int, node_arr.split()))
+        
+    # pre_order print
+    print 'Original bst list: \n %s\n' % str(bst.pre_order_print())
+    erase_keys = raw_input('Please type in key(s) you want to delete:\n')
+    
+    # check whether input is valid
+    try:
+        keys = map(int, erase_keys.split())
+    except ValueError:
+        raise ValueError('invalid input')
+        
+    # check if key exists.
+    for key in keys: 
+        if bst.has_key(key):
+            bst.delete(key)
+        else:
+            print 'key %s not found.' % key
+            
+    print 'New bst list:\n%s\n' % str(bst.pre_order_print())  
\ No newline at end of file
diff --git a/challenge_11/python/shaowen/unit_test.py b/challenge_11/python/shaowen/unit_test.py
new file mode 100644
index 000000000..dc5313f98
--- /dev/null
+++ b/challenge_11/python/shaowen/unit_test.py
@@ -0,0 +1,54 @@
+# -*- coding: utf-8 -*-
+#@author: Shaowen Liu
+
+
+import unittest
+from bst import BST, build_bst
+
+class bst_test(unittest.TestCase):
+    
+    # build proper bst list
+    def test_bst_build_case1(self):
+        arr = [5, 2, 12, 9, 21, 3, -4, 19, 25]
+        bst = build_bst(arr)
+        self.assertEqual(bst.pre_order_print(), [5, 2, -4, 3, 12, 9, 21, 19, 25])
+        
+    def test_bst_build_case2(self):
+        arr = [235, 100, 50, 700, 800, 900, 25, 75, 105, 236]
+        bst = build_bst(arr)
+        self.assertEqual(bst.pre_order_print(), [235,100,50,25,75,105,700,236,800,900])
+
+
+    # test for delete method
+    def test_bst_delete_case1(self):
+        # delete the leaf node
+        arr = [5, 2, -4, 3, 12, 9, 21, 19, 25]
+        bst = build_bst(arr)
+        bst.delete(25)
+        self.assertEqual(bst.pre_order_print(), [5, 2, -4, 3, 12, 9, 21, 19])
+    
+    def test_bst_delete_case2(self):
+        # delete the roof node
+        arr = [5, 2, -4, 3, 12, 9, 21, 19, 25]
+        bst = build_bst(arr)
+        bst.delete(5)
+        self.assertEqual(bst.pre_order_print(), [9, 2, -4, 3, 12, 21, 19, 25])
+        
+    def test_bst_delete_case3(self):
+        # delete the node in the middle of the tree
+        bst_list = [5, 2, -4, 3, 12, 9, 21, 19, 25]
+        bst = build_bst(bst_list)
+        bst.delete(2)
+        self.assertEqual(bst.pre_order_print(), [5, 3, -4, 12, 9, 21, 19, 25])
+
+    def test_bst_delete_case4(self):
+        # delete the node in the middle of the tree
+        arr = [5, 2, -4, 3, 12, 9, 21, 19, 25]
+        bst = build_bst(arr)
+        bst.delete(12)
+        self.assertEqual(bst.pre_order_print(), [5, 2, -4, 3, 19, 9, 21, 25])       
+        
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/challenge_12/python/shaowen/READE.md b/challenge_12/python/shaowen/READE.md
new file mode 100644
index 000000000..9d986b1b8
--- /dev/null
+++ b/challenge_12/python/shaowen/READE.md
@@ -0,0 +1,14 @@
+###Challenge 12
+
+Language Version: Python 2.7.12
+
+
+### Solution
+
+Regex
+
+### How to run
+
+$ python compress.py
+
+(code will detect whether it's compressed or not, and output the opposite.)
diff --git a/challenge_12/python/shaowen/compress.py b/challenge_12/python/shaowen/compress.py
new file mode 100644
index 000000000..06b343db7
--- /dev/null
+++ b/challenge_12/python/shaowen/compress.py
@@ -0,0 +1,37 @@
+# -*- coding: utf-8 -*-
+#@author: Shaowen Liu
+
+import re
+
+def replace_fun(match):
+    duplicated = match.group()
+    return '%s#%d' % (duplicated[0], len(duplicated))
+
+def replace_fun2(match):
+    compressed = match.group()
+    return compressed[0] * int(compressed[2:])
+
+def compress(astring):
+    """compress string"""
+    pattern = re.compile(r'(([a-zA-Z])\2{3,})')
+    
+    return pattern.sub(replace_fun, astring)
+
+def decompress(astring):
+    """decompress string"""
+    pattern = re.compile(r'(\w#\d+)')
+    
+    return pattern.sub(replace_fun2, astring)
+
+if __name__ == '__main__':
+    astring = raw_input('Input string:')
+    
+    if '#' in astring:
+        decompress_str = decompress(astring)
+        print 'Decopressed string:\n%s\n' % decompress_str
+    else:
+        compress_str = compress(astring)
+        print 'Compressed string: \n%s\n' % compress_str
+    
+    
+    
\ No newline at end of file