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"]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList<String>();
        if(root == null) {
            return result;
        }

        String str = "";

        dfs(root, result, str);
        return result;

    }

    private void dfs(TreeNode root, List<String> result, String str) {
        if(str.length() == 0) {
            str = str + Integer.toString(root.val);
        } else {
            str = str + "->" + Integer.toString(root.val);
        }

        if (root.left != null) {
            dfs(root.left, result, str);
        }
        if (root.right != null) {
            dfs(root.right, result, str);
        }

        if (root.left == null && root.right == null) {
            result.add(new String(str));
        }


    }
}