Recursive

class Solution {

public:

bool isSymmetric(TreeNode* root) {

if(!root)

return true;

return isSymmetric(root->left,root->right);

}

bool isSymmetric(TreeNode left, TreeNode right){

if(!left&&!right) //剪枝

return true;

if(!left||!right)

return false;

if(left->val!=right->val)

return false;

return isSymmetric(left->left,right->right) && isSymmetric(left->right,right->left);

}

};