借用二叉树是否相同的代码改动左右孩子相等对应关系，即为是否对称。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
 bool isSameTree(struct TreeNode* p, struct TreeNode* q){
    if(p==NULL&&q==NULL)
        return true;
    if(p==NULL || q == NULL)
        return false;
    if(p->val != q->val)
    {
        return false;
    }
    return isSameTree(p->left,q->right)&&  //此处更改为对称
        isSameTree(p->right,q->left);
}
bool isSymmetric(struct TreeNode* root){
    if(root == NULL)
        return true;
    else
        return isSameTree(root->left, root->right); 
}