101. 对称二叉树

    // 递归
    public boolean isSymmetric(TreeNode root) {
        return isMirror(root, root);
    }

    public boolean isMirror(TreeNode root1, TreeNode root2) {
        if (root1 == null && root2 == null) {
            return true;
        }
        if (root1 == null || root2 == null) {
            return false;
        }
        return (root1.val == root2.val)
                && isMirror(root1.left, root2.right)
                && isMirror(root1.right, root2.left);
    }

    // 迭代
    public boolean isSymmetric2(TreeNode root) {
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        q.add(root);
        while(!q.isEmpty()) {
            TreeNode t1 = q.poll();
            TreeNode t2 = q.poll();
            if (t1 == null && t2 == null) {
                continue;
            }
            if (t1 == null || t2 == null) {
                return false;
            }
            if (t1.val != t2.val) {
                return false;
            }
            q.add(t1.left);
            q.add(t2.right);
            q.add(t1.right);
            q.add(t2.left);
        }
        return true;
    }