TreeNode* sortedListToBST(ListNode* head) {
if (head == nullptr) return nullptr;
vector<int> nums;
while (head != nullptr)
{
nums.emplace_back(head->val);
head = head->next;
}
std::function<TreeNode* (int, int)>
sortedListToBSTHelper = [&](int low, int high) -> TreeNode*
{
if (low > high) return nullptr;
if (low == high)
return new TreeNode(nums[low]);
int mid = low + (high - low)/2;
TreeNode* root = new TreeNode(nums[mid]);
root->left = sortedListToBSTHelper(low, mid-1);
root->right = sortedListToBSTHelper(mid+1, high);
return root;
};
TreeNode* root = sortedListToBSTHelper(0, nums.size()-1);
return root;
}
// !!!!! so important *&. here need to use *& to change the node which cur point
TreeNode* sortedListToBSTHelper(ListNode* &cur, int start, int end){ // update cur: cur is the last node after construction
if(start > end) return NULL; // !!!terminate condition
int mid = start + (end - start) / 2;
TreeNode* ltree = sortedListToBSTHelper(cur,start, mid - 1); // use start and end
TreeNode* subroot = new TreeNode(cur->val); // mid !!!
// cur means the points will be use!!!
subroot->left = ltree;
cur = cur->next;
TreeNode* rtree = sortedListToBSTHelper(cur, mid + 1, end);
subroot->right = rtree;
return subroot;
}
TreeNode* sortedListToBSTHelper(ListNode* head, int nodeNum) {
if(head == NULL || nodeNum == 0) {
return NULL;
}
ListNode* mid = head;
int midNum = nodeNum / 2;
while(midNum--) {
mid = mid->next;
}
TreeNode *root = new TreeNode(mid->val);
root->left = sortedListToBSTHelper(head, nodeNum/2);
root->right = sortedListToBSTHelper(mid->next, nodeNum - nodeNum/2 - 1);
return root;
}
TreeNode* sortedListToBST(ListNode* head) {
int nodeNum = 0;
ListNode* p = head;
while(p) {
p = p->next;
nodeNum++;
}
return sortedListToBSTHelper(head, nodeNum);
}
TreeNode* sortedListToBST(ListNode* head){
int len = 0;
ListNode* cur = head;
while(cur){
len++;
cur = cur->next;
}
sortedListToBSTHelper(head, 0, len - 1);
}
