Adding (root->left!=NULL || root->right!=NULL) judgement to improve efficiency from 6ms to 0ms

```
class Solution {
public:
TreeNode* invertTree(TreeNode* root) {
TreeNode* tmp;
if(root!=NULL && (root->left!=NULL || root->right!=NULL)){//do not forget root==NULL
tmp = root->left;
root->left = root->right;
root->right = tmp;
//above three lines can be written in std::swap(root->left, root->right);
invertTree(root->left);
invertTree(root->right);
}
return root;
}
};
```