算法:对称的二叉树

public class Solution {
    boolean isSymmetrical(TreeNode pRoot) {
        if (pRoot == null) return true;
        return helper(pRoot, pRoot);
    }

    private boolean helper(TreeNode root1, TreeNode root2) {
        if (root1 == null && root2 == null) return true;
        if (root1 == null || root2 == null) return false;
        if (root1.val != root2.val) return false;

        return helper(root1.left, root2.right) && helper(root1.right, root2.left);
    }
}