比较简单的递归思路

boolean isSymmetrical(TreeNode pRoot)
    {
        if(pRoot==null) return true;
        TreeNode pLeft = pRoot.left;
        TreeNode pRight = pRoot.right;
        boolean res = help(pLeft,pRight);
        return res;
    }
    public boolean help(TreeNode pLeft,TreeNode pRight){
        if(pLeft==null && pRight==null )return true;
        //下面是两种不对称的情况：一是左右节点一个为空一个不为空，二是左右节点值不想等
        if(pLeft!=null&&pRight==null) return false;
        if(pLeft==null&&pRight!=null) return false;
        if(pLeft.val != pRight.val){
             return false;
        }else{//递归去判断下面的情况
            return help(pLeft.left,pRight.right)&& help(pLeft.right,pRight.left);
        }

    }