按照題意遞迴即可 因為要遍歷每一個節點所以會存在重複遍歷執行時間增加

class Solution:
    def pathSum(self, root, sum):
        """
        :type root: TreeNode
        :type sum: int
        :rtype: int
        """ 
        res=[0]
        def dfs(root,cursum):
            if not root:
                return 
            if cursum-root.val==0:
                res[0]+=1
            if root.left:
                dfs(root.left,cursum-root.val)
            if root.right:
                dfs(root.right,cursum-root.val)
        if not root:
            return 0
        stack = [root]
        while stack:
            node = stack.pop()
            dfs(node,sum)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return res[0]