leetcode257 Binary Tree Paths

   1
 /   \
2     3
 \
  5

public class leetcode257 {
    private ArrayList<Integer> record = new ArrayList<>();
    private List<String> res = new ArrayList<>();

    public List<String> binaryTreePaths(TreeNode root) {
        helper(root);
        return res;
    }

    private void helper(TreeNode node) {
        if (node != null) {
            //节点加入记录
            record.add(node.val);
            helper(node.left);
            helper(node.right);
            if (node.left == null && node.right == null) {
                //叶子结点，打印输出记录里所有节点.
                String temp = "";
                for (int i = 0; i < record.size(); i++) {
                    if (i == 0) temp += record.get(i);
                    else
                        temp = temp + "->" + record.get(i);
                }
                res.add(temp);
            }
            //此时，该节点左右子树都已经打印输出完毕，可以从记录中删除
            record.remove(record.size() - 1);
        }
    }
}