diff --git a/alien-dictionary/hu6r1s.py b/alien-dictionary/hu6r1s.py
new file mode 100644
index 000000000..e605f22d3
--- /dev/null
+++ b/alien-dictionary/hu6r1s.py
@@ -0,0 +1,44 @@
+from typing import (
+    List,
+)
+
+import heapq
+    heap = [c for c in indegree if indegree[c] == 0]
+    heapq.heapify(heap)
+    res = []
+    
+
+class Solution:
+    """
+    @param words: a list of words
+    @return: a string which is correct order
+    """
+    def alien_order(self, words: List[str]) -> str:
+        # Write your code here
+        adj = {c: set() for word in words for c in word}
+        indegree = {c: 0 for c in adj}
+    
+        for i in range(len(words) - 1):
+            w1, w2 = words[i], words[i+1]
+            minlen = min(len(w1), len(w2))
+            if len(w1) > len(w2) and w1[:minlen] == w2[:minlen]:
+                return ""
+            for j in range(minlen):
+                if w1[j] != w2[j]:
+                    if w2[j] not in adj[w1[j]]:
+                        adj[w1[j]].add(w2[j])
+                        indegree[w2[j]] += 1
+                    break
+        
+        while heap:
+            c = heapq.heappop(heap)
+            res.append(c)
+            for nei in adj[c]:
+                indegree[nei] -= 1
+                if indegree[nei] == 0:
+                    heapq.heappush(heap, nei)
+        
+        if len(res) != len(adj):
+            return ""
+        
+        return "".join(res)
diff --git a/construct-binary-tree-from-preorder-and-inorder-traversal/hu6r1s.py b/construct-binary-tree-from-preorder-and-inorder-traversal/hu6r1s.py
new file mode 100644
index 000000000..5b5c8befd
--- /dev/null
+++ b/construct-binary-tree-from-preorder-and-inorder-traversal/hu6r1s.py
@@ -0,0 +1,16 @@
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        if not inorder:
+            return None
+            
+        val = preorder.pop(0)
+        mid = inorder.index(val)
+        left = self.buildTree(preorder, inorder[:mid])
+        right = self.buildTree(preorder, inorder[mid + 1 :])
+        return TreeNode(val, left, right)
diff --git a/longest-palindromic-substring/hu6r1s.py b/longest-palindromic-substring/hu6r1s.py
new file mode 100644
index 000000000..f528f2ead
--- /dev/null
+++ b/longest-palindromic-substring/hu6r1s.py
@@ -0,0 +1,26 @@
+class Solution:
+    def longestPalindrome(self, s: str) -> str:
+        # if len(s) <= 2:
+        #     return s[0]
+
+        # for i in range(2, len(s)):
+        #     for k in range(len(s)-i):
+        #         if s[k:k+i] == s[k:k+i][::-1]:
+        #             return s[k:k+i]
+
+        max_s, max_e = 0, 0
+
+        for i in range(len(s)):
+            start, end = i, i
+            while 0 <= start and end < len(s) and s[start] == s[end]:
+                if max_e - max_s < end - start:
+                    max_s, max_e = start, end
+                start, end = start - 1, end + 1
+
+            start, end = i, i + 1
+            while 0 <= start and end < len(s) and s[start] == s[end]:
+                if max_e - max_s < end - start:
+                    max_s, max_e = start, end
+                start, end = start - 1, end + 1
+
+        return s[max_s : max_e + 1]
diff --git a/rotate-image/hu6r1s.py b/rotate-image/hu6r1s.py
new file mode 100644
index 000000000..27fe37f4d
--- /dev/null
+++ b/rotate-image/hu6r1s.py
@@ -0,0 +1,20 @@
+class Solution:
+    def rotate(self, matrix: List[List[int]]) -> None:
+        """
+        Do not return anything, modify matrix in-place instead.
+        [0][0] [0][1] [0][2]
+        [1][0] [1][1] [1][2]
+        [2][0] [2][1] [2][2]
+
+        [2][0] [1][0] [0][0]
+        [2][1] [1][1] [0][1]
+        [2][2] [1][2] [0][2]
+        """
+        n = len(matrix)
+        
+        for i in range(n):
+            for j in range(i, n):
+                matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
+        
+        for i in range(n):
+            matrix[i].reverse()
diff --git a/subtree-of-another-tree/hu6r1s.py b/subtree-of-another-tree/hu6r1s.py
new file mode 100644
index 000000000..2804e8207
--- /dev/null
+++ b/subtree-of-another-tree/hu6r1s.py
@@ -0,0 +1,24 @@
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def isSubtree(self, root: Optional[TreeNode], subRoot: Optional[TreeNode]) -> bool:
+        if not subRoot:
+            return True
+        if not root:
+            return False
+
+        def same(root, subRoot):
+            if not root or not subRoot:
+                return not root and not subRoot
+
+            if root.val != subRoot.val:
+                return False
+            return same(root.left, subRoot.left) and same(root.right, subRoot.right)
+
+        if same(root, subRoot):
+            return True
+        return self.isSubtree(root.left, subRoot) or self.isSubtree(root.right, subRoot)