解题思路：

class Solution {public boolean isSymmetric(TreeNode root) {if(root == null) return true;return recur(root.left, root.right);}boolean recur(TreeNode L, TreeNode R) {if (L == null && R == null) return true;if (L == null || R == null || L.val != R.val) return false;return recur(L.left, R.right) && recur(L.right, R.left);}
}