思路一：
递归

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool test(TreeNode* root1,TreeNode* root2)
    {
        if(root1==NULL&&root2==NULL)
        {
            return true;
        }
        if(root1==NULL||root2==NULL)
        {
            return false;
        }
        return (root1->val==root2->val)&&test(root1->left,root2->right)&&test(root1->right,root2->left);
    }
    bool isSymmetric(TreeNode* root) 
    {
        return test(root,root);
    }
};

思路二：
迭代

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) 
    {
        queue<TreeNode*> queue_T;
        TreeNode* root1;
        TreeNode* root2;
        if(root==NULL)
        {
            return true;
        }
        else
        {
            queue_T.push(root);
            queue_T.push(root);
            while(queue_T.size()!=0)
            {
                root1=queue_T.front();
                queue_T.pop();
                root2=queue_T.front();
                queue_T.pop();
                if(root1==NULL&&root2==NULL)
                {
                    continue;
                }
                if(root1==NULL||root2==NULL)
                {
                    return false;
                }
                if(root1->val!=root2->val)
                {
                    return false;
                }
                else
                {
                    queue_T.push(root1->left);
                    queue_T.push(root2->right);
                    queue_T.push(root1->right);
                    queue_T.push(root2->left);
                }
            }
            return true;
        }
    }
};