题目：
题解：
class Solution:def pathSum(self, root: TreeNode, targetSum: int) -> int:prefix = collections.defaultdict(int)prefix[0] = 1def dfs(root, curr):if not root:return 0ret = 0curr += root.valret += prefix[curr - targetSum]prefix[curr] += 1ret += dfs(root.left, curr)ret += dfs(root.right, curr)prefix[curr] -= 1return retreturn dfs(root, 0)