diff --git a/alien-dictionary/printjin-gmailcom.py b/alien-dictionary/printjin-gmailcom.py
new file mode 100644
index 000000000..de66ac1d6
--- /dev/null
+++ b/alien-dictionary/printjin-gmailcom.py
@@ -0,0 +1,29 @@
+import collections
+class Solution:
+    def alien_order(self, words):
+        graph = collections.defaultdict(set)
+        indegree = {}
+        for word in words:
+            for c in word:
+                indegree[c] = 0
+        for i in range(len(words) - 1):
+            w1, w2 = words[i], words[i + 1]
+            min_len = min(len(w1), len(w2))
+            if len(w1) > len(w2) and w1[:min_len] == w2[:min_len]:
+                return "" 
+            for j in range(min_len):
+                if w1[j] != w2[j]:
+                    if w2[j] not in graph[w1[j]]:
+                        graph[w1[j]].add(w2[j])
+                        indegree[w2[j]] += 1
+                    break
+        queue = collections.deque([c for c in indegree if indegree[c] == 0])
+        res = []
+        while queue:
+            c = queue.popleft()
+            res.append(c)
+            for nei in graph[c]:
+                indegree[nei] -= 1
+                if indegree[nei] == 0:
+                    queue.append(nei)
+        return "".join(res) if len(res) == len(indegree) else ""
diff --git a/construct-binary-tree-from-preorder-and-inorder-traversal/printjin-gmailcom.py b/construct-binary-tree-from-preorder-and-inorder-traversal/printjin-gmailcom.py
new file mode 100644
index 000000000..278870291
--- /dev/null
+++ b/construct-binary-tree-from-preorder-and-inorder-traversal/printjin-gmailcom.py
@@ -0,0 +1,14 @@
+class Solution:
+    def buildTree(self, preorder, inorder):
+        inorder_index_map = {val: idx for idx, val in enumerate(inorder)}
+        def build(pre_left, pre_right, in_left, in_right):
+            if pre_left > pre_right:
+                return None
+            root_val = preorder[pre_left]
+            root = TreeNode(root_val)
+            in_root_idx = inorder_index_map[root_val]
+            left_size = in_root_idx - in_left
+            root.left = build(pre_left + 1, pre_left + left_size, in_left, in_root_idx - 1)
+            root.right = build(pre_left + left_size + 1, pre_right, in_root_idx + 1, in_right)
+            return root
+        return build(0, len(preorder) - 1, 0, len(inorder) - 1)
diff --git a/longest-palindromic-substring/printjin-gmailcom.py b/longest-palindromic-substring/printjin-gmailcom.py
new file mode 100644
index 000000000..627f00527
--- /dev/null
+++ b/longest-palindromic-substring/printjin-gmailcom.py
@@ -0,0 +1,13 @@
+class Solution:
+    def longestPalindrome(self, s):
+        def expand(left, right):
+            while left >= 0 and right < len(s) and s[left] == s[right]:
+                left -= 1
+                right += 1
+            return s[left + 1:right]
+        res = ""
+        for i in range(len(s)):
+            odd = expand(i, i)
+            even = expand(i, i + 1)
+            res = max(res, odd, even, key=len)
+        return res
diff --git a/rotate-image/printjin-gmailcom.py b/rotate-image/printjin-gmailcom.py
new file mode 100644
index 000000000..30809951d
--- /dev/null
+++ b/rotate-image/printjin-gmailcom.py
@@ -0,0 +1,8 @@
+class Solution:
+    def rotate(self, matrix):
+        n = len(matrix)
+        for i in range(n):
+            for j in range(i + 1, n):
+                matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
+        for row in matrix:
+            row.reverse()
diff --git a/subtree-of-another-tree/printjin-gmailcom.py b/subtree-of-another-tree/printjin-gmailcom.py
new file mode 100644
index 000000000..9ceefefc5
--- /dev/null
+++ b/subtree-of-another-tree/printjin-gmailcom.py
@@ -0,0 +1,13 @@
+class Solution:
+    def isSubtree(self, root, subRoot):
+        if not root:
+            return False
+        if self.isSameTree(root, subRoot):
+            return True
+        return self.isSubtree(root.left, subRoot) or self.isSubtree(root.right, subRoot)
+    def isSameTree(self, s, t):
+        if not s and not t:
+            return True
+        if not s or not t or s.val != t.val:
+            return False
+        return self.isSameTree(s.left, t.left) and self.isSameTree(s.right, t.right)