Symmetric Tree

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool isSymmetric(TreeNode *root)     {        if(!root)            return true;        return isSymmetric1(root->left,root->right);                                }    bool isSymmetric1(TreeNode *p,TreeNode *q)    {        if(!p&&!q)            return true;        if(!p||!q)            return false;        return p->val==q->val && isSymmetric1(p->left,q->right) && isSymmetric1(p->right,q->left);    }};