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