1.递归

public boolean isSymmetric(TreeNode root) {
        if(root == null){
            return true;
        }
        return deepCheck(root.left,root.right);
    }

    boolean deepCheck(TreeNode left, TreeNode right){
        //递归的终止条件是两个节点都为空
        //或者两个节点中有一个为空
        //或者两个节点的值不相等
        if(left == null && right == null){
            return true;
        }
        if(left == null || right == null){
            return false;
        }
        if(left.val  != right.val){
            return false;
        }

        //再递归的比较，左节点的左孩子，和右节点的右孩子
        //以及比较 左节点的右孩子 和右节点的左孩子
        return deepCheck(left.left,right.right) && deepCheck(left.right,right.left);
    }

2.循环迭代

public boolean isSymmetric(TreeNode root) {
        Queue<TreeNode> q = new LinkedList<>();
        TreeNode u = root.left;
        TreeNode v  = root.right;
        if(root == null || (u==null && v==null)){
            return true;
        }
        q.offer(u);
        q.offer(v);
        while (!q.isEmpty()){
            u = q.poll();
            v = q.poll();
            if(u==null && v==null){
                continue;
            }
            if( (u==null || v==null) || (u.val != v.val) ){
                return false;
            }
            q.offer(u.left);
            q.offer(v.right);
            
            q.offer(u.right);
            q.offer(v.left);
        }
        return true;
    }