leetcode 257[easy]-----Binary Tree Paths

難度：easy Given a binary tree, return all root-to-leaf paths. For example, given the following binary tree: 1 / \ 2 3 \ 5 All root-to-leaf paths are: ["1->2->5", "1->3"] 思路：開始學習兩叉樹，仍然是以練帶學

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