[lintcode] - 391 Number of Airplanes in the Sky

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easy难度的题，基本上是不允许错的，也必须10分钟内做出来。
思路很简单，递归至最深处将结果保存即可。

/** * @author: decaywood * @date: 2016/1/27 9:30 *  * Given a binary tree, return all root-to-leaf paths. *  * Example * Given the following binary tree: *  *           1 *         /   \ *        2     3 *         \ *          5 * All root-to-leaf paths are: *  * [ *  "1->2->5", *  "1->3" * ] * */public class BinaryTreePaths {    public List<String> binaryTreePaths(TreeNode root) {        List<String> list = new ArrayList<>();        getPath(list, root, "");        return list;    }    private void getPath(List<String> res, TreeNode root, String path) {        if (root == null) return;        path = path + "->" + root.val;        if(root.left == null && root.right == null) res.add(path.substring(2));        getPath(res, root.left, path);        getPath(res, root.right, path);    }}

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