对称二叉树

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    boolean isSymmetrical(TreeNode pRoot) {
        if (null == pRoot) {
            return true;
        }
        return helper(pRoot.left, pRoot.right);
    }
    
    boolean helper(TreeNode left, TreeNode right) {
        if (null == left && null == right) {
            return true;
        } else if (null == left || null == right) {
            return false;
        } else if (left.val != right.val) {
            return false;
        }
        return helper(left.left, right.right) && helper(left.right, right.left);
    }
}