26. Symmetric Tree

/**
 * Definition for binary tree
 * 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;
        return isSymmetric(root.left, root.right);
    }
   
    private boolean isSymmetric(TreeNode tree1, TreeNode tree2) {
        if (tree1==null && tree2==null) return true;
        if ((tree1==null && tree2!=null) || (tree1!=null && tree2==null)) return false;
        if (tree1.val!=tree2.val) return false;
        else return isSymmetric(tree1.left, tree2.right) && isSymmetric(tree1.right, tree2.left);
    }
}