LeetCode 101. Symmetric Tree

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isSymmetric(TreeNode root) {    if (root == null) return true;    else return values(root.left, true).equals(values(root.right, false));    }        public String values(TreeNode root, boolean fromLeft) {    if (root == null) return "";    String v = "" + root.val;    if (fromLeft) {    if (root.left != null) v += values(root.left, true);    else v += "a";    if (root.right != null) v += values(root.right, true);    else v += "a";    } else {    if (root.right != null) v += values(root.right, false);    else v += "a";    if (root.left != null) v += values(root.left, false);    else v += "a";    }    return v;    }}