Update 437. Path Sum III.py

Co-Authored-By: Antim-IWP <203163676+Antim-IWP@users.noreply.github.com>
Co-Authored-By: Shiwangi Srivastava <174641070+IamShiwangi@users.noreply.github.com>

Author: 3 people

Solution/437. Path Sum III/437. Path Sum III.py

@@ -0,0 +1,21 @@
+# 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 pathSum(self, root: Optional[TreeNode], targetSum: int) -> int:
+        def dfs(node, s):
+            if node is None:
+                return 0
+            s += node.val
+            ans = cnt[s - targetSum]
+            cnt[s] += 1
+            ans += dfs(node.left, s)
+            ans += dfs(node.right, s)
+            cnt[s] -= 1
+            return ans
+
+        cnt = Counter({0: 1})
+        return dfs(root, 0)