使用 O(1) 额外内存删除二叉树

这是一个 naive 的做法：

void deleteTreeRec(TreeNode *root){
   if (root == NULL) return;
   deleteTreeRec(root->left);
   deleteTreeRec(root->right);
   cout << "Deleting node " << root->data << endl;
   delete root;
}

O(1) 空间的做法如下：

把树的左侧节点看成一条链，我们的目的就是不断把当前 root 节点的右子树挪到这条链的末尾，最终可以把这棵树展平为一条链。

而在展开的过程中我们可以删除这条链的头节点（即root节点），最后可以把这条链删除干净。

void deleteTree(TreeNode* root) {
    TreeNode* tail = root;
    while (root != nullptr){
        // update tail
        while (tail->left != nullptr) {
            tail = tail->left;
        }
        // move right to the end of the "list"
        // needs to happen before retrieving next, since the node may only have a right subtree
        tail->left = root->right;
        // need to retrieve the data about the next
        TreeNode* next = root->left;
        delete root;
        root = next;
    }
}