Convert Sorted List to Binary Search Tree

Convert Sorted List to Binary Search TreeOct 3 '123397
/ 9589

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

 

刚开始想用课本上的构造平衡二叉树的方法，当不满足时，回溯，旋转，结果在运行大的数据时，超时。

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
void insertBST(TreeNode* &root, int val)
{
if(root == NULL)
{
root = new TreeNode(val);
}
else if(val > root->val)
{
insertBST(root->right, val);
}
else if(val < root->val)
{
insertBST(root->left, val);
}
}
void turnLeft(TreeNode* &root)
{

TreeNode* tempCR = root->right->left;
root->right->left = root;
root = root->right;
root->left->right = tempCR;
}
int adjust(TreeNode* &root)
{
if(root)
{
int hl = adjust(root->left);
int hr = adjust(root->right);
if(hr > hl + 1)
{
turnLeft(root);
return max(hl, hr);
}
return max(hl, hr)+1;
}
return 0;
}
TreeNode *sortedListToBST(ListNode *head) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if(head == NULL)
return NULL;
TreeNode* root = NULL;
while(head)
{
insertBST(root, head->val);
adjust(root);
head = head->next;
}
return root;
}
};

 

后来，想直接在中间节点分开，递归构造。复杂度应该为nlogn

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode* buildTree(ListNode* head, int len)
{
if(head && len > 0)
{
int l = len/2;
ListNode* p = head;
int count = 0;
while(p && count < l)
{
count ++;
p = p->next;
}
TreeNode* tre = new TreeNode(p->val);
TreeNode* lc = buildTree(head, count);
TreeNode* lr = buildTree(p->next, len - count -1);
tre->left = lc;
tre->right = lr;
return tre;
}
return NULL;
}
TreeNode *sortedListToBST(ListNode *head) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
int len = 0;
ListNode* p = head;
while(p)
{
len ++;
p = p ->next;
}
return buildTree(head,len);
}
};