Given a binary tree, return all root-to-leaf paths.

public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList<String>();
        if (root == null) return result;
        helper(root, "", result);
        return result;
    }

    public void helper(TreeNode root, String path, List<String> result) {
        if (root.left == null && root.right == null) {
            result.add(new String(path + root.val));//leaf node必须单独处理
            return;
        }
        if (root.left != null) helper(root.left, path + root.val + "->" , result);
        if (root.right != null) helper(root.right, path + root.val + "->", result);
    }
}

class Solution(object):
    def binaryTreePaths(self, root):
        """
        :type root: TreeNode
        :rtype: List[str]
        """
        if not root:
            return []

        ret = []
        self.helper(root, [], ret)
        return ret

    def helper(self, root, curr, ret):
        if not root.left and not root.right:
            ret.append("->".join(curr + [str(root.val)]))
            return
        if root.left:
            self.helper(root.left, curr + [str(root.val)], ret)
        if root.right:
            self.helper(root.right, curr + [str(root.val)], ret)