Invert binary tree, reverse the inorder list
1. Invert or Mirror Binary Tree
2. Reverse inorder list [Nice Property]
Node* invert(Node* root){
    if(root){
        Node* right = invert(root->left);
        Node* left  = invert(root->right);
        root->left = left ;
        root->right = right;
        return root;
    }
    return NULL;
}

Node* invert_top_down(Node* root){
    if(root){
        Node* left = root->left;
        root->left = root->right;
        root->right = left;
        invert_top_down(root->left);
        invert_top_down(root->right);
        return root;
    }
    return NULL;
}