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Author: wuduhren

problems/most-frequent-subtree-sum.py

@@ -17,4 +17,24 @@ def dfs(node):
 """
 Time: O(N)
 Space: O(N)
-"""
+"""
+
+"""
+Time: O(N)
+Space: O(N)
+
+Throught the getSubtreeSum(root), we will also count subtreeSum of each node.
+"""
+class Solution(object):
+    def findFrequentTreeSum(self, root):
+        def getSubtreeSum(node):
+            if not node: return 0
+            subtreeSum = node.val+getSubtreeSum(node.left)+getSubtreeSum(node.right)
+            memo[subtreeSum] += 1
+            return subtreeSum
+        
+        
+        memo = collections.Counter()
+        getSubtreeSum(root)
+        mostFrequenctCount = max([memo[subtreeSum] for subtreeSum in memo])
+        return [subtreeSum for subtreeSum in memo if memo[subtreeSum]==mostFrequenctCount]

problems/verify-preorder-serialization-of-a-binary-tree.py

@@ -0,0 +1,14 @@
+"""
+Time: O(N)
+Space: O(N) for creating the list preorder.split(',')
+"""
+class Solution(object):
+    def isValidSerialization(self, preorder):
+        slot = 1
+        
+        for c in preorder.split(','):
+            slot -= 1 #each elemet consumes a slot
+            if slot<0: return False
+            if c!='#': slot += 2 #each non-null node also create 2 slot
+            
+        return slot==0 #all slots should be fill