思路：题目需要记录从根节点开始走的路径，无疑选用前序遍历，用一个数组paths 记录走过的节点信息，遇到叶子节点就用另一个list记录下路径，回溯时删掉paths尾节点即可

class Solution {
     public List<String> binaryTreePaths(TreeNode root) {

        List<String> list = new ArrayList<>(); //处理的结果
        if(root == null) {
            return list;
        }
        List<Integer> paths = new ArrayList<>(); //当前路径顺序
        func(root, paths, list);
        return list;

    }
    public void func(TreeNode root, List<Integer> paths, List<String> list) {
        //前序遍历，中
        paths.add(root.val);

        //如果是叶子节点，就记录下路径
        if(root.left==null && root.right == null) {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < paths.size()-1; i++) {
                sb.append(paths.get(i)).append("->");
            }
            sb.append(paths.get(paths.size()-1));
            list.add(sb.toString());
            //return;

        }
        //左，不是叶子节点就一直递归深入
        if(root.left != null) {
            func(root.left, paths, list);
            paths.remove(paths.size()-1);//结束一轮递归要回退一步

        }
        //右
        if(root.right != null) {
            func(root.right, paths, list);
            paths.remove(paths.size()-1);

        }
    }

}