剑指 Offer 28：对称的二叉树

1
   / \
  2   2
 / \ / \
3  4 4  3

1
   / \
  2   2
   \   \
   3    3

输入：root = [1,2,2,3,4,4,3]
输出：true

输入：root = [1,2,2,null,3,null,3]
输出：false

class Solution {
public:
    bool dfs(TreeNode*left,TreeNode* right){
        if(!(left||right)) return true;
        if(!(left&&right)) return false;
        if(left->val!=right->val) return false;
        return dfs(left->left,right->right)&&dfs(left->right,right->left);
    }
    bool isSymmetric(TreeNode* root) {
        if(!root) return true;
        return dfs(root->left,root->right);
    }
};