0257：二叉树的所有路径

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class Solution:
    def binaryTreePaths(self, root: Optional[TreeNode]) -> List[str]:
        def dfs(u):
            path.append(str(u.val))
            if not u.left and not u.right:
                res.append('->'.join(path))
            for v in [u.left,u.right]:
                if v:
                    dfs(v)
            path.pop()
        res,path = [],[]
        dfs(root)
        return res