101. Symmetric Tree

和same tree是一样的，就是same tree里比较的是两个树的同一位置，现在是看做这个树和这棵树的翻转

    public boolean isSymmetric(TreeNode root) {
        return isSameTree(root, root);
    }
    
    private boolean isSameTree(TreeNode p, TreeNode q) {
        if(p == null && q == null) {
            return true;
        }
        if(p == null || q == null) {
            return false;
        }
        if(p.val != q.val) {
            return false;
        }
        return isSameTree(p.left, q.right) && isSameTree(p.right, q.left);
    }