递归法：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        return isMirror(root,root);    
    }

    bool isMirror(TreeNode* t1,TreeNode* t2)
    {
        if(t1==nullptr&&t2==nullptr)
        return true;

        if(t1==nullptr||t2==nullptr)
        return false;

        return((t1->val==t2->val)&&isMirror(t1->right,t2->left)&&isMirror(t1->left,t2->right));
    }
};