void flattenHelper(struct TreeNode* root, struct TreeNode **start, struct TreeNode **end){
if (root == NULL) {
*start = *end = NULL;
return;
}
struct TreeNode *lstart, *lend, *rstart, *rend;
lstart = lend = rstart = rend = NULL;
flattenHelper(root->left, &lstart, &lend);
flattenHelper(root->right, &rstart, &rend);
root->left = NULL;
if (lend != NULL) {
root->right = lstart;
lend->right = rstart;
}
else {
root->right = rstart;
}
*start = root;
if (rend != NULL) {
*end = rend;
}
else if (lend != NULL) {
*end = lend;
}
else {
*end = root;
}
}
std::pair<Node *, Node *> flattenHelper(Node *root) {
if (root != NULL) {
if (root->left == NULL && root->right == NULL) {
return {root, root};
} else {
auto left_tree = flattenHelper(root->left);
auto right_tree = flattenHelper(root->right);
if (left_tree.first == NULL) {
root->left = NULL;
left_tree.first = root;
} else {
left_tree.second->right = root;
root->left = left_tree.second;
}
if (right_tree.first == NULL) {
root->right = NULL;
right_tree.second = root;
} else {
right_tree.first->left = root;
root->right = right_tree.first;
}
return {left_tree.first, right_tree.second};
}
}
return {NULL, NULL};
}
void flatten(struct TreeNode* root) {
struct TreeNode *start, *end;
start = end = NULL;
flattenHelper(root, &start, &end);
}
Node * flatten(Node *root) {
if (root) {
return flattenHelper(root).first;
}
return NULL;
}
