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Author: Chris Wu

path-sum-iii.py

@@ -0,0 +1,50 @@
+from collections import Counter
+
+class Solution(object):
+    def pathSum(self, root, sum):
+        def helper(node, sum_from_root, record):
+            sum_from_root += node.val
+            sum_to_p = sum_from_root-sum
+            self.ans += record[sum_to_p]
+
+            record[sum_from_root] += 1 #1
+            if node.left:
+                helper(node.left, sum_from_root, record)
+            if node.right:
+                helper(node.right, sum_from_root, record)
+            record[sum_from_root] -= 1 #2
+        
+        self.ans = 0
+        if not root: return self.ans
+        record = Counter()
+        record[0] = 1 #3
+        helper(root, 0, record)
+        return self.ans
+
+"""
+This answer is inspired by @gabbu
+Some variable and function:
+1. `self.ans` stores the number of paths that sum to a given value.
+2. `helper()` will traverse the tree and increase `self.ans` along the way.
+3. `sum_from_root` is the sum from the `node` to the `root`. Note that, there maybe multiple ways to get to a node.
+4. `record` is the number of ways that paths sums up to a number. In short, `record[any_sum]` you get **number of ways to have any_sum**.
+
+0.
+`record` is the hardest part to understand, you need to see the main idea below first.
+Imagine we are now at a node in the tree, N. The sum of the path from root to N (including `N.val`) is `sum_from_root`.
+Imagine a predecessor of N, we call it P. The sum of the path root->P (including N.val) is `sum_to_p`. Given a node N, there maybe multiple or no P.
+`sum_to_p + sum(P->N) == sum_from_root`, thus `sum(P->N) == sum_from_root - sum_to_p`
+This problem we are looking for **the number of ways P->N, where `sum == sum(P->N)`** (where `sum == sum_from_root - sum_to_p`).
+Thus, we are basically looking for the number of ways root->P, where `sum_to_p == sum_from_root - sum`.
+So that is why we have `record`. We can find the number of ways root->P by record[sum_to_p] (record[sum_from_root - sum]).
+
+1.
+Maintain the `record`.
+
+2.
+Remove the case from the `record` after all the children are done calculating.
+This prevent other node counts the irerlevant `record`.
+
+Time complexity is O(N). Since we only traverse each node once.
+Space complexity O(N). Since in the worst case, we might go N-level of recursion.
+"""