对称二叉树

递归

class Solution{
public:
    bool isSymmetric(TreeNode* root) {
        if(!root) return true;
        return m(root->left,root->right);
        }
        bool m(TreeNode*r1,TreeNode*r2)
        {
            bool x,y;
            if(r1==nullptr)
            {
                if(r2==nullptr)return true;
                return false;   
            }
            else if(r1!=nullptr)
            {
                if(r2==nullptr)return false;
                if(r1->val!=r2->val)return false;
                x=m(r1->left,r2->right);
                y=m(r1->right,r2->left);
            
        }
        return x&&y;
        }
};