Added code for Binary tree path sum

bt_path_sum.py

@@ -0,0 +1,11 @@
+def pathSum(self, root: TreeNode, target_sum: int) -> int:
+    def dfs(node: TreeNode, sums: List[int]) -> int:
+        if not node:
+            return 0
+        sums = [s + node.val for s in sums] + [node.val]
+        ans = sums.count(target_sum)
+        ans += dfs(node.left, sums) if node.left else 0
+        ans += dfs(node.right, sums) if node.right else 0
+        return ans
+
+    return dfs(root, [])